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Archive for the ‘Advanced’ Category

Advance/Troubleshooting/Error-Bug Fixing

My favorite 5 linux TOP features/flags β€¦.

Posted by FatDBA on July 23, 2021

Hi Guys,

Recently I started the ‘Top 5’ series where I share my top 5 features in any particular tool or product. Last time I did for SQL Developer command line (SQLcl), this time it will be about one of Linux’s favorite tool ‘TOP‘.

So, without any particular order, below are my top 5 picks of options/flags available with the tool.

c : To get absolute path of the commands currently executing. This will be quite helpful if you want to know from where the command/executable is running. I have used this option lot of the times in multiple performance battles that I have won πŸ™‚ Look at the screen below and in to column COMMAND.

z: will display running process in RED color which may help you to identified running process easily.

1: To get usage details per CPU core.

SHIFT+i : To disable the IRIX (default mode is ON) mode, help you to understand – Why Process CPU % Usage larger than Total CPU Time ?

E: To get memory details in KB, MB & GB, press ‘E’ each time. So, no need to separately call ‘free’ command.

I know this is all about top 5 picks, but here goes my 6th top (kinda reserve option) πŸ™‚

V: Tree Mode, to display the processes in a parent child hierarchy.

Hope It Helped
Prashant Dixit

Posted in Advanced, troubleshooting | Tagged: , | Leave a Comment »

How to fix/handle Wraparound problem in PostgreSQL ?

Posted by FatDBA on July 21, 2021

Hi Guys,

In continuation from where I left in my last post on simulating wraparounds in PostgreSQL, this post is all about managing the mess that we spilled πŸ™‚ Though this was a synthetic/artificial test and as soon you close or exit SESSION 1 where you’ve opened a BEGIN block/transaction and ran select txid_current() to block autovacuum, it (autovacuum) will automatically kick-in and perform a quick full vacuum on all of the databases in the cluster. You have to remember that even if you have autovacuum turned off, it will still start in emergency mode.

So, this is what we got earlier at the time we hit by the wraparound problem when we tried to execute a DML and it says something about stopping postmaster and vacuum databases in single-user mode; What is that ? – We will see that next ….

enterprisedb=#
enterprisedb=# SELECT datname
, age(datfrozenxid)
, current_setting('autovacuum_freeze_max_age')
FROM pg_database
 ORDER BY 2 DESC;
 
   datname    |    age     | current_setting
--------------+------------+-----------------
 postgres     | 2146483647 | 200000000
 edb          | 2146483647 | 200000000
 template1    | 2146483647 | 200000000
 template0    | 2146483647 | 200000000
 immortal     | 2146483647 | 200000000
 darkthrone   | 2146483647 | 200000000
 immortal1    | 2146483647 | 200000000
 mayhem       | 2146483647 | 200000000
 murduk       | 2146483647 | 200000000
 burzum       | 2146483647 | 200000000
 gorgoroth    | 2146483647 | 200000000
 repmgr       | 2146483647 | 200000000
 enterprisedb | 2146483647 | 200000000
(13 rows)



enterprisedb=# update pgbench_history set aid=65555 where aid=64444;
ERROR:  database is not accepting commands to avoid wraparound data loss in database "postgres"
HINT:  Stop the postmaster and vacuum that database in single-user mode.
You might also need to commit or roll back old prepared transactions, or drop stale replication slots.
enterprisedb=#
enterprisedb=#

Okay, coming back to the point, how to handle the situation now when you’ve already hit the wraparound. So, first thing you have to manually shut down the database cluster, if it’s still up.

-bash-4.2$ pwd
/usr/edb/as11/bin

-bash-4.2$ /usr/edb/as11/bin/pg_ctl stop -D /var/lib/edb/as11/data
waiting for server to shut down.... done
server stopped
-bash-4.2$

Next, you need to run VACUUM FULL for each of the database (one-by-one using single mode) through stand-alone backend mode. For that you have to call postgres (if you’re on open source) or edb-postgres utility. I am on EDB PostgreSQL 11 so will be calling edb-postgres and execute VACUUM FULL.

-bash-4.2$ pwd
/usr/edb/as11/bin

-bash-4.2$
-bash-4.2$ ls *postgre*
edb-postgres
-bash-4.2$

-bash-4.2$ ./edb-postgres --single -D /var/lib/edb/as11/data enterprisedb

PostgreSQL stand-alone backend 11.11.20
backend> vacuum full;
2021-07-20 11:07:49 IST LOG:  statement: vacuum full;

backend>

It’s done for ‘enterprisedb’ database. Next will do it for ‘postgresql’ database, but this time I will use some stand-alone backend related command to assure we are connected with the right database and to enable runtime stats or verbose.

-bash-4.2$ ./edb-postgres --single -D /var/lib/edb/as11/data postgres

PostgreSQL stand-alone backend 11.11.20
backend>
backend> select current_database();
2021-07-20 13:45:25 IST LOG:  statement: select current_database();

         1: current_database    (typeid = 19, len = 64, typmod = -1, byval = f)
        ----
         1: current_database = "postgres"       (typeid = 19, len = 64, typmod = -1, byval = f)
        ----
backend> 


backend> SELECT set_config('log_statement_stats', 'on', false);
2021-07-20 13:48:55 IST LOG:  statement: SELECT set_config('log_statement_stats', 'on', false);

         1: set_config  (typeid = 25, len = -1, typmod = -1, byval = f)
        ----
         1: set_config = "on"   (typeid = 25, len = -1, typmod = -1, byval = f)
        ----
backend>

-- Now when we have enabled logging of stats, let's execute the VACUUM to see runtime stats
backend> vacuum full;
2021-07-20 13:49:29 IST LOG:  statement: vacuum full;

2021-07-20 13:49:30 IST LOG:  QUERY STATISTICS
2021-07-20 13:49:30 IST DETAIL:  ! system usage stats:
        !       0.557302 s user, 0.426659 s system, 120.357278 s elapsed
        !       [0.583998 s user, 0.455579 s system total]
        !       21756 kB max resident size
        !       224/402352 [240/402824] filesystem blocks in/out
        !       0/2558 [0/6201151] page faults/reclaims, 0 [0] swaps
        !       0 [0] signals rcvd, 0/0 [0/0] messages rcvd/sent
        !       982/2 [1005/3] voluntary/involuntary context switches
2021-07-20 13:58:39 IST STATEMENT:  vacuum full;

backend>

Okay, same way we have to run VACUUM FULL on all of the databases and restart PostgreSQL cluster.
Once all of the databases are done and after restart, connect with the database and you will see the TXID or datfrozenid has been reset and you are all good.

enterprisedb=# SELECT datname
, age(datfrozenxid)
, current_setting('autovacuum_freeze_max_age')
FROM pg_database
 ORDER BY 2 DESC;

    datname    | age | current_setting
--------------+-----+-----------------
postgres     | 1369 | 200000000
edb          | 1272 | 200000000
template1    | 1170 | 200000000
template0    | 1071 | 200000000
immortal     |  974 | 200000000
darkthrone   |  877 | 200000000
immortal1    |  780 | 200000000
mayhem       |  585 | 200000000
murduk       |  487 | 200000000
burzum       |  388 | 200000000
gorgoroth    |  291 | 200000000
repmgr       |  194 | 200000000
enterprisedb |   97 | 200000000

You’re all set!

Hope It Helped!
Prashant Dixit

Posted in Advanced, troubleshooting | Tagged: , , , | Leave a Comment »

How to simulate the deadly, infamous, misunderstood & complex ‘Transaction Wraparound Problem’ in PostgreSQL …

Posted by FatDBA on July 20, 2021

Hi Guys,

Last week one of my good friend asked me an interesting question – ‘How to replicate the transaction wraparound problem’ in PostgreSQL ?

He has a demo/POC that he needs to do for one of his customer who is interested to see how it looks like in reality and how ‘deadly’ is the problem and how to fix this. He scanned almost everything on the Google, but unfortunately found nothing available or pre-cooked …

So, this might be the only post available on the internet explaining how to exactly simulate the transaction wraparound in the PostgreSQL Database. This one was really interesting and I have never tried this before as I thought I will never see transaction ID wraparound error πŸ™‚ . So, before I jump in to the demo, we need to take care of few things. Use a system with good resources, I have performed this demo on an EDB PostgreSQL 11.11.20 database with 8 CPUs, 31 GB RAM, and it took me around 9 hours to exhaust all transactionIDs and reach the limit of 2,000,000,000.

Would like to thank Nikolay Samokhvalov for his help! πŸ™‚

Before I start, let’s do this one more time – What is a transaction ID and why it wrapsaround ?
The PostgreSQL transaction control mechanism assigns a transaction ID to every row that is modified in the database; these IDs control the visibility of that row to other concurrent transactions. Txid wraparound is a problem due to MVCC. MVCC relies on being able to take the txids of two transactions and determine which of the transactions came first.
A PostgreSQL database can have two billion ‘in-flight’ unvacuumed transactions before PostgreSQL takes dramatic action to avoid data loss. If the number of unvacuumed transactions reaches (2^31 – 10,000,000), the log starts warning that vacuuming is needed. If the number of unvacuumed transactions reaches 2,000,000,000, PostgreSQL sets the database to read-only mode and requires an offline, single-user, standalone vacuum.

Note: You have to carefully monitor your database pg_log, archiving locations as this activity will generate huge WALs. At the same time, keep an eye on the size of your log directory, specially if you are capturing all statements.

We will open multiple parallel sessions to simulate the workload and monitor the progress.

SESSION 1:

In the first session, we will first connect with the database and set idle_in_transaction_session_timeout to value 0 and start a BEGIN block and execute select txid_current(); this is to block autovacuum, as simply turning it off wouldn’t help because emegency mode kick in even with `autovacuum = off.

The idle_in_transaction_session_timeout parameter terminate any session with an open transaction that has been idle for longer than the specified duration in milliseconds. This allows any locks held by that session to be released and the connection slot to be reused; it also allows tuples visible only to this transaction to be vacuumed. So, we will have to set it to value 0 that will disable this feature.

-bash-4.2$ psql -p 5444
psql.bin (11.11.20)
Type "help" for help.

enterprisedb=# SET idle_in_transaction_session_timeout = 0;
SET
enterprisedb=#  BEGIN;
BEGIN
enterprisedb=# select txid_current();
 txid_current
--------------
         1266

enterprisedb=#
enterprisedb=#

SESSION 2:

Now, I will create a file with current transaction ID written in to it and will call the pgbench. I am doing something like below to call pgbench, below are the explanations on flags used and why I used them ..

pgbench -U enterprisedb enterprisedb -f wl.sql -c8 -j8 -T100000 -rn -P60

-f: to read from a file
-c8: 8 clients
-j8: 8 threads/jobs (adjust it as per your CPU count)
-T100000: seconds to run the test for this many seconds, rather than a fixed number of transactions per client
-rn: where r is to report the average per-statement latency (execution time from the perspective of the client) of each command after the benchmark finishes. Whereas n is to perform no vacuuming before running the test. This option is necessary if you are running a custom test scenario that does not include the standard tables pgbench_accounts, pgbench_branches, pgbench_history, and pgbench_tellers.
-P60: is to show progress report every sec seconds

-bash-4.2$ 
-bash-4.2$ echo 'select txid_current();' > wl.sql
-bash-4.2$ 
-bash-4.2$
-bash-4.2$ /usr/edb/as11/bin/pgbench -U enterprisedb enterprisedb -f wl.sql -c8 -j8 -T100000 -rn -P60
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progress: 9900.0 s, 51219.8 tps, lat 0.156 ms stddev 0.260
progress: 9960.0 s, 52235.8 tps, lat 0.153 ms stddev 0.184
progress: 10020.0 s, 51904.5 tps, lat 0.154 ms stddev 0.186
progress: 10080.0 s, 52066.8 tps, lat 0.154 ms stddev 0.183
progress: 10140.0 s, 52212.5 tps, lat 0.153 ms stddev 0.209
progress: 10200.0 s, 52402.8 tps, lat 0.153 ms stddev 0.186
progress: 10260.0 s, 52194.8 tps, lat 0.153 ms stddev 0.178
progress: 10320.0 s, 52117.3 tps, lat 0.153 ms stddev 0.179
progress: 10380.0 s, 52060.5 tps, lat 0.154 ms stddev 0.181
progress: 10440.0 s, 52043.1 tps, lat 0.154 ms stddev 0.183
progress: 10500.0 s, 52358.5 tps, lat 0.153 ms stddev 0.178
progress: 10560.0 s, 51361.2 tps, lat 0.156 ms stddev 0.179
progress: 10620.0 s, 51655.7 tps, lat 0.155 ms stddev 0.184
progress: 10680.0 s, 51501.1 tps, lat 0.155 ms stddev 0.176
progress: 10740.0 s, 51315.4 tps, lat 0.156 ms stddev 0.182
progress: 10800.0 s, 52344.5 tps, lat 0.153 ms stddev 0.181
progress: 10860.0 s, 51687.0 tps, lat 0.155 ms stddev 0.182
progress: 10920.0 s, 51150.2 tps, lat 0.156 ms stddev 0.179
progress: 10980.0 s, 51341.9 tps, lat 0.156 ms stddev 0.181
progress: 11040.0 s, 51074.0 tps, lat 0.157 ms stddev 0.184
progress: 11100.0 s, 51645.4 tps, lat 0.155 ms stddev 0.277
progress: 11160.0 s, 51984.6 tps, lat 0.154 ms stddev 0.182
progress: 11220.0 s, 51993.4 tps, lat 0.154 ms stddev 0.181
progress: 11280.0 s, 51894.9 tps, lat 0.154 ms stddev 0.183
progress: 11340.0 s, 52007.2 tps, lat 0.154 ms stddev 0.186
progress: 11400.0 s, 52255.9 tps, lat 0.153 ms stddev 0.188
progress: 11460.0 s, 52221.2 tps, lat 0.153 ms stddev 0.183
progress: 11520.0 s, 51990.5 tps, lat 0.154 ms stddev 0.181
progress: 11580.0 s, 52138.3 tps, lat 0.153 ms stddev 0.178
progress: 11640.0 s, 51258.2 tps, lat 0.156 ms stddev 0.178
progress: 11700.0 s, 52268.0 tps, lat 0.153 ms stddev 0.179
progress: 11760.0 s, 51838.4 tps, lat 0.154 ms stddev 0.180
progress: 11820.0 s, 52079.2 tps, lat 0.154 ms stddev 0.185
progress: 11880.0 s, 52531.4 tps, lat 0.152 ms stddev 0.175
progress: 11940.0 s, 51711.7 tps, lat 0.155 ms stddev 0.260
progress: 12000.0 s, 52020.7 tps, lat 0.154 ms stddev 0.180
progress: 12060.0 s, 51990.1 tps, lat 0.154 ms stddev 0.241
progress: 12120.0 s, 51367.6 tps, lat 0.156 ms stddev 0.193
progress: 12180.0 s, 51316.0 tps, lat 0.156 ms stddev 0.187
progress: 12240.0 s, 50819.0 tps, lat 0.157 ms stddev 0.190
progress: 12300.0 s, 51614.6 tps, lat 0.155 ms stddev 0.182
progress: 12360.0 s, 51500.6 tps, lat 0.155 ms stddev 0.187
progress: 12420.0 s, 51715.8 tps, lat 0.155 ms stddev 0.259
progress: 12480.0 s, 51805.0 tps, lat 0.154 ms stddev 0.183
progress: 12540.0 s, 51846.2 tps, lat 0.154 ms stddev 0.183
progress: 12600.0 s, 51829.6 tps, lat 0.154 ms stddev 0.179
progress: 12660.0 s, 51411.1 tps, lat 0.156 ms stddev 0.221
progress: 12720.0 s, 51728.5 tps, lat 0.155 ms stddev 0.187
progress: 12780.0 s, 52170.5 tps, lat 0.153 ms stddev 0.181
progress: 12840.0 s, 51985.8 tps, lat 0.154 ms stddev 0.182
progress: 12900.0 s, 51723.3 tps, lat 0.155 ms stddev 0.181
progress: 12960.0 s, 52324.2 tps, lat 0.153 ms stddev 0.184
progress: 13020.0 s, 51795.5 tps, lat 0.154 ms stddev 0.189
progress: 13080.0 s, 51547.3 tps, lat 0.155 ms stddev 0.373
progress: 13140.0 s, 51592.4 tps, lat 0.155 ms stddev 0.231
progress: 13200.0 s, 52004.8 tps, lat 0.154 ms stddev 0.223
progress: 13260.0 s, 52118.3 tps, lat 0.153 ms stddev 0.184
progress: 13320.0 s, 51781.1 tps, lat 0.154 ms stddev 0.183
progress: 13380.0 s, 50621.8 tps, lat 0.158 ms stddev 0.196
progress: 13440.0 s, 50671.8 tps, lat 0.158 ms stddev 0.321
progress: 13500.0 s, 51389.4 tps, lat 0.156 ms stddev 0.181
progress: 13560.0 s, 51653.2 tps, lat 0.155 ms stddev 0.186
progress: 13620.0 s, 51491.7 tps, lat 0.155 ms stddev 0.181
progress: 13680.0 s, 51639.4 tps, lat 0.155 ms stddev 0.183
progress: 13740.0 s, 51385.3 tps, lat 0.156 ms stddev 0.188
progress: 13800.0 s, 51640.2 tps, lat 0.155 ms stddev 0.177
progress: 13860.0 s, 51820.7 tps, lat 0.154 ms stddev 0.176
progress: 13920.0 s, 52041.9 tps, lat 0.154 ms stddev 0.179
progress: 13980.0 s, 52001.0 tps, lat 0.154 ms stddev 0.178
progress: 14040.0 s, 52189.3 tps, lat 0.153 ms stddev 0.176
progress: 14100.0 s, 51897.9 tps, lat 0.154 ms stddev 0.179
progress: 14160.0 s, 52066.4 tps, lat 0.154 ms stddev 0.180
progress: 14220.0 s, 52257.9 tps, lat 0.153 ms stddev 0.178
progress: 14280.0 s, 51850.1 tps, lat 0.154 ms stddev 0.210
......
.........
...........

SESSION 3:

Now in this third session we will monitor the database to see what’s going on. You can do something like below to check where we are going.

Below SQL will show you the ‘datfrozenxid‘ which is a value for all transaction IDs before this one have been replaced with a permanent (“frozen”) transaction ID in this database. This is used to track whether the database needs to be vacuumed in order to prevent transaction ID wraparound. And to see ‘autovacuum_freeze_max_age‘ that is age at which to autovacuum a table to prevent transaction ID wraparound.

Do a watch (\watch 10) on the command to see the progress and the play live πŸ™‚

SELECT datname
    , age(datfrozenxid)
    , current_setting('autovacuum_freeze_max_age') 
FROM pg_database 
ORDER BY 2 DESC;


   datname    |   age   | current_setting
--------------+---------+-----------------
 postgres     | 3058531 | 200000000
 edb          | 3058531 | 200000000
 template1    | 3058531 | 200000000
 template0    | 3058531 | 200000000
 immortal     | 3058531 | 200000000
 darkthrone   | 3058531 | 200000000
 immortal1    | 3058531 | 200000000
 mayhem       | 3058531 | 200000000
 marduk       | 3058531 | 200000000
 burzum       | 3058531 | 200000000
 gorgoroth    | 3058531 | 200000000
 repmgr       | 3058531 | 200000000
 enterprisedb | 3058531 | 200000000
(13 rows)

I have used below query to check where we are going, current XIDs, how close we are to our target of wraparound ..

enterprisedb=# WITH max_age AS ( 
    SELECT 2000000000 as max_old_xid
        , setting AS autovacuum_freeze_max_age 
        FROM pg_catalog.pg_settings 
        WHERE name = 'autovacuum_freeze_max_age' )
, per_database_stats AS ( 
    SELECT datname
        , m.max_old_xid::int
        , m.autovacuum_freeze_max_age::int
        , age(d.datfrozenxid) AS oldest_current_xid 
    FROM pg_catalog.pg_database d 
    JOIN max_age m ON (true) 
    WHERE d.datallowconn ) 
SELECT max(oldest_current_xid) AS oldest_current_xid
    , max(ROUND(100*(oldest_current_xid/max_old_xid::float))) AS percent_towards_wraparound
    , max(ROUND(100*(oldest_current_xid/autovacuum_freeze_max_age::float))) AS percent_towards_emergency_autovac 
FROM per_database_stats


\watch 10

 oldest_current_xid | percent_towards_wraparound | percent_towards_emergency_autovac
--------------------+----------------------------+-----------------------------------
          289007513 |                         14 |                               145
(1 row)



 oldest_current_xid | percent_towards_wraparound | percent_towards_emergency_autovac
--------------------+----------------------------+-----------------------------------
          290077770 |                         15 |                               145
(1 row)


 oldest_current_xid | percent_towards_wraparound | percent_towards_emergency_autovac
--------------------+----------------------------+-----------------------------------
          292467983 |                         15 |                               146
(1 row)


                     Mon 19 Jul 2021 08:38:01 PM IST (every 10s)

 oldest_current_xid | percent_towards_wraparound | percent_towards_emergency_autovac
--------------------+----------------------------+-----------------------------------
          342714029 |                         17 |                               171
(1 row)

                     Mon 19 Jul 2021 08:38:11 PM IST (every 10s)

 oldest_current_xid | percent_towards_wraparound | percent_towards_emergency_autovac
--------------------+----------------------------+-----------------------------------
          343223530 |                         17 |                               172
(1 row)


                     Mon 19 Jul 2021 09:17:45 PM IST (every 10s)

 oldest_current_xid | percent_towards_wraparound | percent_towards_emergency_autovac
--------------------+----------------------------+-----------------------------------
          406185619 |                         20 |                               203
(1 row)


 oldest_current_xid | percent_towards_wraparound | percent_towards_emergency_autovac
--------------------+----------------------------+-----------------------------------
          421779698 |                         21 |                               211
(1 row)

                     Mon 19 Jul 2021 09:28:10 PM IST (every 10s)

 oldest_current_xid | percent_towards_wraparound | percent_towards_emergency_autovac
--------------------+----------------------------+-----------------------------------
          438446236 |                         22 |                               219
(1 row)



                     Mon 19 Jul 2021 11:03:03 PM IST (every 10s)

 oldest_current_xid | percent_towards_wraparound | percent_towards_emergency_autovac
--------------------+----------------------------+-----------------------------------
          733479765 |                         37 |                               367
(1 row)



 oldest_current_xid | percent_towards_wraparound | percent_towards_emergency_autovac
--------------------+----------------------------+-----------------------------------
          763852636 |                         38 |                               382
(1 row)


                     Mon 19 Jul 2021 11:33:03 PM IST (every 10s)

 oldest_current_xid | percent_towards_wraparound | percent_towards_emergency_autovac
--------------------+----------------------------+-----------------------------------
          826485794 |                         41 |                               413
(1 row)


                     Tue 20 Jul 2021 12:18:07 AM IST (every 10s)

 oldest_current_xid | percent_towards_wraparound | percent_towards_emergency_autovac
--------------------+----------------------------+-----------------------------------
          966692697 |                         48 |                               483
(1 row)


 oldest_current_xid | percent_towards_wraparound | percent_towards_emergency_autovac
--------------------+----------------------------+-----------------------------------
         1107823196 |                         55 |                               554
 

                     Tue 20 Jul 2021 01:09:18 AM IST (every 10s)

 oldest_current_xid | percent_towards_wraparound | percent_towards_emergency_autovac
--------------------+----------------------------+-----------------------------------
         1125393734 |                         56 |                               563
(1 row)


                     Tue 20 Jul 2021 01:43:11 AM IST (every 10s)

 oldest_current_xid | percent_towards_wraparound | percent_towards_emergency_autovac
--------------------+----------------------------+-----------------------------------
         1230984982 |                         62 |                               615
(1 row)





-- LETS SEE WHAT'S HAPPENING ON THE DATABASE 
enterprisedb=# \x
Expanded display is on.
enterprisedb=#
enterprisedb=# select * from pg_stat_activity;


-[ RECORD 4 ]----+---------------------------------
datid            | 16384
datname          | enterprisedb
pid              | 24056
usesysid         | 10
usename          | enterprisedb
application_name | psql.bin
client_addr      |
client_hostname  |
client_port      | -1
backend_start    | 20-JUL-21 09:45:44.833016 +05:30
xact_start       | 20-JUL-21 09:45:57.749892 +05:30
query_start      | 20-JUL-21 09:46:01.042977 +05:30
state_change     | 20-JUL-21 09:46:01.047161 +05:30
wait_event_type  | Client
wait_event       | ClientRead
state            | idle in transaction
backend_xid      | 1195
backend_xmin     |
query            | select txid_current();
backend_type     | client backend
-[ RECORD 5 ]----+---------------------------------
datid            | 16384
datname          | enterprisedb
pid              | 25884
usesysid         | 10
usename          | enterprisedb
application_name | psql.bin
client_addr      |
client_hostname  |
client_port      | -1
backend_start    | 20-JUL-21 09:47:36.766729 +05:30
xact_start       | 20-JUL-21 09:52:18.941776 +05:30
query_start      | 20-JUL-21 09:52:18.941776 +05:30
state_change     | 20-JUL-21 09:52:18.941778 +05:30
wait_event_type  |
wait_event       |
state            | active
backend_xid      |
backend_xmin     | 1195
query            | select * from pg_stat_activity;
backend_type     | client backend
-[ RECORD 6 ]----+---------------------------------
datid            | 16384
datname          | enterprisedb
pid              | 27150
usesysid         | 10
usename          | enterprisedb
application_name | pgbench
client_addr      |
client_hostname  |
client_port      | -1
backend_start    | 20-JUL-21 09:49:56.70284 +05:30
xact_start       | 20-JUL-21 09:52:18.941429 +05:30
query_start      | 20-JUL-21 09:52:18.941429 +05:30
state_change     | 20-JUL-21 09:52:18.94143 +05:30
wait_event_type  | IPC
wait_event       | ClogGroupUpdate
state            | active
backend_xid      | 4526420
backend_xmin     |
query            | select txid_current();
backend_type     | client backend
-[ RECORD 7 ]----+---------------------------------
datid            | 16384
datname          | enterprisedb
pid              | 27151
usesysid         | 10
usename          | enterprisedb
application_name | pgbench
client_addr      |
client_hostname  |
client_port      | -1
backend_start    | 20-JUL-21 09:49:56.704062 +05:30
xact_start       |
query_start      | 20-JUL-21 09:52:18.943293 +05:30
state_change     | 20-JUL-21 09:52:18.943411 +05:30
wait_event_type  | Client
wait_event       | ClientRead
state            | idle
backend_xid      |
backend_xmin     |
query            | select txid_current();
backend_type     | client backend
-[ RECORD 8 ]----+---------------------------------
datid            | 16384
datname          | enterprisedb
pid              | 27153
usesysid         | 10
usename          | enterprisedb
application_name | pgbench
client_addr      |
client_hostname  |
client_port      | -1
backend_start    | 20-JUL-21 09:49:56.7067 +05:30
xact_start       |
query_start      | 20-JUL-21 09:52:18.943218 +05:30
state_change     | 20-JUL-21 09:52:18.943334 +05:30
wait_event_type  |
wait_event       |
state            | idle
backend_xid      |
backend_xmin     |
query            | select txid_current();
backend_type     | client backend
-[ RECORD 9 ]----+---------------------------------
datid            | 16384
datname          | enterprisedb
pid              | 27154
usesysid         | 10
usename          | enterprisedb
application_name | pgbench
client_addr      |
client_hostname  |
client_port      | -1
backend_start    | 20-JUL-21 09:49:56.707948 +05:30
xact_start       | 20-JUL-21 09:52:18.941116 +05:30
query_start      | 20-JUL-21 09:52:18.941116 +05:30
state_change     | 20-JUL-21 09:52:18.941117 +05:30
wait_event_type  | IPC
wait_event       | ClogGroupUpdate
state            | active
backend_xid      | 4526414
backend_xmin     |
query            | select txid_current();
backend_type     | client backend
-[ RECORD 10 ]---+---------------------------------
datid            | 16384
datname          | enterprisedb
pid              | 27152
usesysid         | 10
usename          | enterprisedb
application_name | pgbench
client_addr      |
client_hostname  |
client_port      | -1
backend_start    | 20-JUL-21 09:49:56.710155 +05:30
xact_start       | 20-JUL-21 09:52:18.943403 +05:30
query_start      | 20-JUL-21 09:52:18.943403 +05:30
state_change     | 20-JUL-21 09:52:18.943405 +05:30
wait_event_type  |
wait_event       |
state            | active
backend_xid      |
backend_xmin     |
query            | select txid_current();
backend_type     | client backend
-[ RECORD 11 ]---+---------------------------------
datid            | 16384
datname          | enterprisedb
pid              | 27155
usesysid         | 10
usename          | enterprisedb
application_name | pgbench
client_addr      |
client_hostname  |
client_port      | -1
backend_start    | 20-JUL-21 09:49:56.710785 +05:30
xact_start       | 20-JUL-21 09:52:18.941493 +05:30
query_start      | 20-JUL-21 09:52:18.941493 +05:30
state_change     | 20-JUL-21 09:52:18.941494 +05:30
wait_event_type  | IPC
wait_event       | ClogGroupUpdate
state            | active
backend_xid      | 4526421
backend_xmin     |
query            | select txid_current();
backend_type     | client backend
-[ RECORD 12 ]---+---------------------------------
datid            | 16384
datname          | enterprisedb
pid              | 27156
usesysid         | 10
usename          | enterprisedb
application_name | pgbench
client_addr      |
client_hostname  |
client_port      | -1
backend_start    | 20-JUL-21 09:49:56.711762 +05:30
xact_start       |
query_start      | 20-JUL-21 09:52:18.942582 +05:30
state_change     | 20-JUL-21 09:52:18.942704 +05:30
wait_event_type  | Client
wait_event       | ClientRead
state            | idle
backend_xid      |
backend_xmin     |
query            | select txid_current();
backend_type     | client backend
-[ RECORD 13 ]---+---------------------------------
datid            | 16384
datname          | enterprisedb
pid              | 27157
usesysid         | 10
usename          | enterprisedb
application_name | pgbench
client_addr      |
client_hostname  |
client_port      | -1
backend_start    | 20-JUL-21 09:49:56.711944 +05:30
xact_start       | 20-JUL-21 09:52:18.943345 +05:30
query_start      | 20-JUL-21 09:52:18.943345 +05:30
state_change     | 20-JUL-21 09:52:18.943346 +05:30
wait_event_type  | Client
wait_event       | ClientRead
state            | active
backend_xid      | 4526447
backend_xmin     | 1195
query            | select txid_current();
backend_type     | client backend
-[ RECORD 14 ]---+---------------------------------



SESSION 4:

Open another session to check log files, if they are reporting any hints, warnings or errors
After ~1 hour of test run, I have started seeing below warnings in the log files.

2021-07-19 20:15:26 IST LOG:  statement: select txid_current();
2021-07-19 20:15:26 IST LOG:  statement: select txid_current();
2021-07-19 20:15:26 IST LOG:  statement: select txid_current();
2021-07-19 20:15:26 IST LOG:  statement: select txid_current();
2021-07-19 20:15:26 IST WARNING:  oldest xmin is far in the past
2021-07-19 20:15:26 IST HINT:  Close open transactions soon to avoid wraparound problems.
        You might also need to commit or roll back old prepared transactions, or drop stale replication slots.
2021-07-19 20:15:26 IST LOG:  statement: select txid_current();
2021-07-19 20:15:26 IST LOG:  statement: select txid_current();
2021-07-19 20:15:26 IST LOG:  statement: select txid_current();
2021-07-19 20:15:26 IST LOG:  statement: select txid_current();
2021-07-19 20:15:26 IST LOG:  statement: select txid_current();
2021-07-19 20:15:26 IST LOG:  statement: select txid_current();
2021-07-19 20:15:26 IST LOG:  statement: select txid_current();
2021-07-19 20:15:26 IST LOG:  statement: select txid_current();
2021-07-19 20:15:26 IST WARNING:  oldest xmin is far in the past
2021-07-19 20:15:26 IST HINT:  Close open transactions soon to avoid wraparound problems.
        You might also need to commit or roll back old prepared transactions, or drop stale replication slots.
2021-07-19 20:15:26 IST LOG:  statement: select txid_current();
2021-07-19 20:15:26 IST LOG:  statement: select txid_current();
2021-07-19 20:15:26 IST LOG:  statement: select txid_current();
2021-07-19 20:15:26 IST LOG:  statement: select txid_current();

After ~ 3 hours of test run, the warnings becoming more prominent.

-bash-4.2$ grep -Rinw /var/lib/edb/as11/data/log/ -e 'wraparound'
/var/lib/edb/as11/data/log/edb-2021-07-19_195300.log:17711:2021-07-19 19:53:01 IST HINT:  Close open transactions soon to avoid wraparound problems.
/var/lib/edb/as11/data/log/edb-2021-07-19_195300.log:20705:2021-07-19 19:53:01 IST HINT:  Close open transactions soon to avoid wraparound problems.
/var/lib/edb/as11/data/log/edb-2021-07-19_195300.log:21160:2021-07-19 19:53:01 IST HINT:  Close open transactions soon to avoid wraparound problems.
/var/lib/edb/as11/data/log/edb-2021-07-19_195300.log:21650:2021-07-19 19:53:01 IST HINT:  Close open transactions soon to avoid wraparound problems.
/var/lib/edb/as11/data/log/edb-2021-07-19_195300.log:22179:2021-07-19 19:53:01 IST HINT:  Close open transactions soon to avoid wraparound problems.
/var/lib/edb/as11/data/log/edb-2021-07-19_195300.log:22694:2021-07-19 19:53:01 IST HINT:  Close open transactions soon to avoid wraparound problems.
/var/lib/edb/as11/data/log/edb-2021-07-19_195300.log:23229:2021-07-19 19:53:01 IST HINT:  Close open transactions soon to avoid wraparound problems.
/var/lib/edb/as11/data/log/edb-2021-07-19_195300.log:23776:2021-07-19 19:53:01 IST HINT:  Close open transactions soon to avoid wraparound problems.
/var/lib/edb/as11/data/log/edb-2021-07-19_195300.log:24316:2021-07-19 19:53:01 IST HINT:  Close open transactions soon to avoid wraparound problems.
/var/lib/edb/as11/data/log/edb-2021-07-19_195300.log:24840:2021-07-19 19:53:01 IST HINT:  Close open transactions soon to avoid wraparound problems.
/var/lib/edb/as11/data/log/edb-2021-07-19_195300.log:25391:2021-07-19 19:53:01 IST HINT:  Close open transactions soon to avoid wraparound problems.
/var/lib/edb/as11/data/log/edb-2021-07-19_195300.log:25927:2021-07-19 19:53:01 IST HINT:  Close open transactions soon to avoid wraparound problems.
/var/lib/edb/as11/data/log/edb-2021-07-19_195300.log:27533:2021-07-19 19:53:01 IST HINT:  Close open transactions soon to avoid wraparound problems.
/var/lib/edb/as11/data/log/edb-2021-07-19_195300.log:28135:2021-07-19 19:53:01 IST HINT:  Close open transactions soon to avoid wraparound problems.
/var/lib/edb/as11/data/log/edb-2021-07-19_195300.log:32986:2021-07-19 19:53:01 IST HINT:  Close open transactions soon to avoid wraparound problems.
/var/lib/edb/as11/data/log/edb-2021-07-19_195300.log:33581:2021-07-19 19:53:01 IST HINT:  Close open transactions soon to avoid wraparound problems.
/var/lib/edb/as11/data/log/edb-2021-07-19_195300.log:17711:2021-07-19 19:53:01 IST HINT:  Close open transactions soon to avoid wraparound problems.
/var/lib/edb/as11/data/log/edb-2021-07-19_195300.log:20705:2021-07-19 19:53:01 IST HINT:  Close open transactions soon to avoid wraparound problems.
/var/lib/edb/as11/data/log/edb-2021-07-19_195300.log:21160:2021-07-19 19:53:01 IST HINT:  Close open transactions soon to avoid wraparound problems.
/var/lib/edb/as11/data/log/edb-2021-07-19_195300.log:21650:2021-07-19 19:53:01 IST HINT:  Close open transactions soon to avoid wraparound problems.
/var/lib/edb/as11/data/log/edb-2021-07-19_195300.log:22179:2021-07-19 19:53:01 IST HINT:  Close open transactions soon to avoid wraparound problems.
/var/lib/edb/as11/data/log/edb-2021-07-19_195300.log:22694:2021-07-19 19:53:01 IST HINT:  Close open transactions soon to avoid wraparound problems.
/var/lib/edb/as11/data/log/edb-2021-07-19_195300.log:23229:2021-07-19 19:53:01 IST HINT:  Close open transactions soon to avoid wraparound problems.
/var/lib/edb/as11/data/log/edb-2021-07-19_195300.log:23776:2021-07-19 19:53:01 IST HINT:  Close open transactions soon to avoid wraparound problems.
/var/lib/edb/as11/data/log/edb-2021-07-19_195300.log:24316:2021-07-19 19:53:01 IST HINT:  Close open transactions soon to avoid wraparound problems.
/var/lib/edb/as11/data/log/edb-2021-07-19_195300.log:24840:2021-07-19 19:53:01 IST HINT:  Close open transactions soon to avoid wraparound problems.
/var/lib/edb/as11/data/log/edb-2021-07-19_195300.log:25391:2021-07-19 19:53:01 IST HINT:  Close open transactions soon to avoid wraparound problems.
/var/lib/edb/as11/data/log/edb-2021-07-19_195300.log:25927:2021-07-19 19:53:01 IST HINT:  Close open transactions soon to avoid wraparound problems.
/var/lib/edb/as11/data/log/edb-2021-07-19_195300.log:27533:2021-07-19 19:53:01 IST HINT:  Close open transactions soon to avoid wraparound problems.
/var/lib/edb/as11/data/log/edb-2021-07-19_195300.log:28135:2021-07-19 19:53:01 IST HINT:  Close open transactions soon to avoid wraparound problems.
/var/lib/edb/as11/data/log/edb-2021-07-19_195300.log:32986:2021-07-19 19:53:01 IST HINT:  Close open transactions soon to avoid wraparound problems.
/var/lib/edb/as11/data/log/edb-2021-07-19_195300.log:33581:2021-07-19 19:53:01 IST HINT:  Close open transactions soon to avoid wraparound problems.

Now, let’s monitor system stats too to see overall load, processes their commands etc.

top - 00:19:39 up 159 days,  7:11,  6 users,  load average: 10.31, 9.97, 9.95
Tasks: 399 total,   9 running, 390 sleeping,   0 stopped,   0 zombie
%Cpu(s): 60.0 us, 38.4 sy,  0.0 ni,  1.6 id,  0.1 wa,  0.0 hi,  0.0 si,  0.0 st
KiB Mem : 32780168 total, 14568928 free,  1259976 used, 16951264 buff/cache
KiB Swap: 16777212 total, 16685044 free,    92168 used. 23083820 avail Mem

  PID USER      PR  NI    VIRT    RES    SHR S %CPU %MEM     TIME+ COMMAND
21831 enterpr+  20   0  591764   5228   2152 S 16.8  0.0 417:57.53 /usr/edb/as11/bin/pgbench -U enterprisedb enterprisedb -f wl.sql -c8 -j8 -T100000 -rn -P+
21845 enterpr+  20   0 8667644  24604  22256 R  9.7  0.1 209:53.07 postgres: enterprisedb enterprisedb [local] SELECT
21840 enterpr+  20   0 8667644  24584  22244 R  9.4  0.1 210:36.23 postgres: enterprisedb enterprisedb [local] idle
21844 enterpr+  20   0 8667644  24876  22528 R  9.4  0.1 209:38.54 postgres: enterprisedb enterprisedb [local] idle
21841 enterpr+  20   0 8667644  24836  22496 R  8.8  0.1 209:15.88 postgres: enterprisedb enterprisedb [local] SELECT
21847 enterpr+  20   0 8667644  24600  22252 S  8.8  0.1 206:16.87 postgres: enterprisedb enterprisedb [local] idle
21846 enterpr+  20   0 8667644  24604  22256 S  8.5  0.1 210:41.59 postgres: enterprisedb enterprisedb [local] idle
20421 enterpr+  20   0  259688   2124    632 R  7.1  0.0 146:27.66 postgres: logger
21843 enterpr+  20   0 8667644  24588  22248 R  6.6  0.1 210:05.38 postgres: enterprisedb enterprisedb [local] idle
21842 enterpr+  20   0 8667644  24600  22260 R  6.6  0.1 209:48.66 postgres: enterprisedb enterprisedb [local] idle
20426 enterpr+  20   0 8666684  18736  17208 R  4.0  0.1 102:43.53 postgres: walwriter
 6858 polkitd   20   0  620236  18044   4428 S  0.9  0.1  15940:23 /usr/lib/polkit-1/polkitd --no-debug

SESSION 2:

To my surprise, after ~8 hours of test run, the session screen where I have pgbench running, started throwing some WARNINGs to perform vacuum within specified number of transactions to avoid database shutdown. It kept on throwing those direct warnings for almost an hour before it stopped processing pgbench requests and started aborting workers with a message ‘ERROR: database is not accepting commands to avoid wraparound data loss in database “postgres”.

In the end it will present you the overall pgbench stats where it fired 2146483646 statements in total.

WARNING:  database "postgres" must be vacuumed within 5092920 transactions
HINT:  To avoid a database shutdown, execute a database-wide VACUUM in that database.
You might also need to commit or roll back old prepared transactions, or drop stale replication slots.
WARNING:  database "postgres" must be vacuumed within 5092920 transactions
HINT:  To avoid a database shutdown, execute a database-wide VACUUM in that database.
You might also need to commit or roll back old prepared transactions, or drop stale replication slots.
WARNING:  database "postgres" must be vacuumed within 5092917 transactions
HINT:  To avoid a database shutdown, execute a database-wide VACUUM in that database.
....
......
HINT:  To avoid a database shutdown, execute a database-wide VACUUM in that database.
You might also need to commit or roll back old prepared transactions, or drop stale replication slots.
WARNING:  database "postgres" must be vacuumed within 3178805 transactions
HINT:  To avoid a database shutdown, execute a database-wide VACUUM in that database.
You might also need to commit or roll back old prepared transactions, or drop stale replication slots.
WARNING:  database "postgres" must be vacuumed within 3178808 transactions
HINT:  To avoid a database shutdown, execute a database-wide VACUUM in that database.
You might also need to commit or roll back old prepared transactions, or drop stale replication slots.
WARNING:  database "postgres" must be vacuumed within 3178804 transactions
HINT:  To avoid a database shutdown, execute a database-wide VACUUM in that database.
You might also need to commit or roll back old prepared transactions, or drop stale replication slots.
WARNING:  database "postgres" must be vacuumed within 3178803 transactions
HINT:  To avoid a database shutdown, execute a database-wide VACUUM in that database.
You might also need to commit or roll back old prepared transactions, or drop stale replication slots.
WARNING:  database "postgres" must be vacuumed within 3178801 transactions
HINT:  To avoid a database shutdown, execute a database-wide VACUUM in that database.
You might also need to commit or roll back old prepared transactions, or drop stale replication slots.
WARNING:  database "postgres" must be vacuumed within 3178799 transactions
HINT:  To avoid a database shutdown, execute a database-wide VACUUM in that database.
......
........
You might also need to commit or roll back old prepared transactions, or drop stale replication slots.
WARNING:  database "postgres" must be vacuumed within 1000002 transactions
HINT:  To avoid a database shutdown, execute a database-wide VACUUM in that database.
You might also need to commit or roll back old prepared transactions, or drop stale replication slots.
WARNING:  database "postgres" must be vacuumed within 1000002 transactions
HINT:  To avoid a database shutdown, execute a database-wide VACUUM in that database.
You might also need to commit or roll back old prepared transactions, or drop stale replication slots.
client 0 aborted in command 0 (SQL) of script 0; ERROR:  database is not accepting commands to avoid wraparound data loss in database "postgres"
HINT:  Stop the postmaster and vacuum that database in single-user mode.
You might also need to commit or roll back old prepared transactions, or drop stale replication slots.

client 1 aborted in command 0 (SQL) of script 0; ERROR:  database is not accepting commands to avoid wraparound data loss in database "postgres"
HINT:  Stop the postmaster and vacuum that database in single-user mode.
You might also need to commit or roll back old prepared transactions, or drop stale replication slots.

client 7 aborted in command 0 (SQL) of script 0; ERROR:  database is not accepting commands to avoid wraparound data loss in database "postgres"
HINT:  Stop the postmaster and vacuum that database in single-user mode.
You might also need to commit or roll back old prepared transactions, or drop stale replication slots.

client 6 aborted in command 0 (SQL) of script 0; ERROR:  database is not accepting commands to avoid wraparound data loss in database "postgres"
HINT:  Stop the postmaster and vacuum that database in single-user mode.
You might also need to commit or roll back old prepared transactions, or drop stale replication slots.

client 2 aborted in command 0 (SQL) of script 0; ERROR:  database is not accepting commands to avoid wraparound data loss in database "postgres"
HINT:  Stop the postmaster and vacuum that database in single-user mode.
You might also need to commit or roll back old prepared transactions, or drop stale replication slots.

client 4 aborted in command 0 (SQL) of script 0; ERROR:  database is not accepting commands to avoid wraparound data loss in database "postgres"
HINT:  Stop the postmaster and vacuum that database in single-user mode.
You might also need to commit or roll back old prepared transactions, or drop stale replication slots.

WARNING:  database "postgres" must be vacuumed within 1000005 transactions
HINT:  To avoid a database shutdown, execute a database-wide VACUUM in that database.
You might also need to commit or roll back old prepared transactions, or drop stale replication slots.
client 3 aborted in command 0 (SQL) of script 0; ERROR:  database is not accepting commands to avoid wraparound data loss in database "postgres"
HINT:  Stop the postmaster and vacuum that database in single-user mode.
You might also need to commit or roll back old prepared transactions, or drop stale replication slots.

client 5 aborted in command 0 (SQL) of script 0; ERROR:  database is not accepting commands to avoid wraparound data loss in database "postgres"
HINT:  Stop the postmaster and vacuum that database in single-user mode.
You might also need to commit or roll back old prepared transactions, or drop stale replication slots.

transaction type: wl.sql
scaling factor: 1
query mode: simple
number of clients: 8
number of threads: 8
duration: 100000 s
number of transactions actually processed: 2146483646
latency average = 0.159 ms
latency stddev = 1.102 ms
tps = 50437.366776 (including connections establishing)
tps = 50437.386645 (excluding connections establishing)
statement latencies in milliseconds:
         0.171  select txid_current();
You have new mail in /var/spool/mail/enterprisedb
-bash-4.2$

SESSION 3:

Lets check our screen 3 to see where we are in terms on oldest current XID and percent towards wraparound, and I see XID numbers are not increasing and we have reached 107% towards wraparound (strange, why it goes beyond 100%).

enterprisedb=# WITH max_age AS ( 
    SELECT 2000000000 as max_old_xid
        , setting AS autovacuum_freeze_max_age 
        FROM pg_catalog.pg_settings 
        WHERE name = 'autovacuum_freeze_max_age' )
, per_database_stats AS ( 
    SELECT datname
        , m.max_old_xid::int
        , m.autovacuum_freeze_max_age::int
        , age(d.datfrozenxid) AS oldest_current_xid 
    FROM pg_catalog.pg_database d 
    JOIN max_age m ON (true) 
    WHERE d.datallowconn ) 
SELECT max(oldest_current_xid) AS oldest_current_xid
    , max(ROUND(100*(oldest_current_xid/max_old_xid::float))) AS percent_towards_wraparound
    , max(ROUND(100*(oldest_current_xid/autovacuum_freeze_max_age::float))) AS percent_towards_emergency_autovac 
FROM per_database_stats;

(1 row)
                     Tue 20 Jul 2021 06:57:51 AM IST (every 10s)

 oldest_current_xid | percent_towards_wraparound | percent_towards_emergency_autovac
--------------------+----------------------------+-----------------------------------
         2146483647 |                        107 |                              1073
(1 row)


                     Tue 20 Jul 2021 06:58:01 AM IST (every 10s)

 oldest_current_xid | percent_towards_wraparound | percent_towards_emergency_autovac
--------------------+----------------------------+-----------------------------------
         2146483647 |                        107 |                              1073
(1 row)




enterprisedb=# SELECT datname
, age(datfrozenxid)
, current_setting('autovacuum_freeze_max_age')
FROM pg_database
 ORDER BY 2 DESC;

   datname    |    age     | current_setting
--------------+------------+-----------------
 postgres     | 2146483647 | 200000000
 edb          | 2146483647 | 200000000
 template1    | 2146483647 | 200000000
 template0    | 2146483647 | 200000000
 immortal     | 2146483647 | 200000000
 darkthrone   | 2146483647 | 200000000
 immortal1    | 2146483647 | 200000000
 mayhem       | 2146483647 | 200000000
 murduk       | 2146483647 | 200000000
 burzum       | 2146483647 | 200000000
 gorgoroth    | 2146483647 | 200000000
 repmgr       | 2146483647 | 200000000
 enterprisedb | 2146483647 | 200000000
(13 rows)

Let’s connect with the database and see what happens when we try to do both SELECT & UPDATE statements …

enterprisedb=# \dt+
                            List of relations
 Schema |       Name       | Type  |    Owner     |  Size   | Description
--------+------------------+-------+--------------+---------+-------------
 public | pgbench_accounts | table | enterprisedb | 13 MB   |
 public | pgbench_branches | table | enterprisedb | 40 kB   |
 public | pgbench_history  | table | enterprisedb | 40 kB   |
 public | pgbench_tellers  | table | enterprisedb | 40 kB   |
 public | scale_data       | table | enterprisedb | 6720 MB |
(5 rows)

enterprisedb=# select * from pgbench_history;
 tid | bid |  aid  | delta |           mtime           | filler
-----+-----+-------+-------+---------------------------+--------
   2 |   1 | 64444 | -1096 | 19-JUL-21 18:41:15.655738 |
   3 |   1 | 18460 | -4688 | 19-JUL-21 18:41:15.668699 |
   2 |   1 | 92762 | -4669 | 19-JUL-21 18:41:15.670741 |
   2 |   1 | 55081 |  1817 | 19-JUL-21 18:41:15.672719 |
   5 |   1 | 47743 |   803 | 19-JUL-21 18:41:15.674666 |
   5 |   1 | 23899 | -3910 | 19-JUL-21 18:41:15.67664  |
  10 |   1 | 33384 |  4005 | 19-JUL-21 18:41:15.678631 |
   6 |   1 | 88383 | -4257 | 19-JUL-21 18:41:15.680629 |
   8 |   1 | 87562 |   200 | 19-JUL-21 18:41:15.682612 |
   5 |   1 |  3957 | -2407 | 19-JUL-21 18:41:15.684595 |
(10 rows)

enterprisedb=#
enterprisedb=# update pgbench_history set aid=65555 where aid=64444;
ERROR:  database is not accepting commands to avoid wraparound data loss in database "postgres"
HINT:  Stop the postmaster and vacuum that database in single-user mode.
You might also need to commit or roll back old prepared transactions, or drop stale replication slots.
enterprisedb=#
enterprisedb=#
enterprisedb=#

Alright, so with transaction wraparound is already there, we only be able to run SELECT queries, but DMLS will fail with a message printed to stop the database and vacuum it in single-user mode.

So, what we have learned

  • Database will keep on running and accepting READ ONLY (SELECT) requests, error will be thrown in case if DMLs tried.
  • 2,000,000,000 seems a huge number, but in case of busy transactional database, it’s just a matter of few hours.
  • AUTOVACUUMING is good thing, at least for this scenario, let’s not discuss about its bugs and other problems.
  • Add VACUUMING a routine activity else you will soon find yourself in a big mess.

The easiest way to get the highest TXID age value back down is to force a vacuum on the entire database cluster. And the best way to do this cluster-wide vacuum is the vacuumdb binary utility that comes with PostgreSQL. I will cover it in my next post as this post is all about simulating wraparounds.

Hope It Helped!
Prashant Dixit

Posted in Advanced, troubleshooting | Tagged: , , | 2 Comments »

New names to RecoveryWalAll and RecoveryWalStream wait events in PostgreSQL 13

Posted by FatDBA on July 9, 2021

Hi Guys,

Recently while working on PostgreSQL Version 13 I have identified few new changes related with the wait events, particularly related with recovery in order to make the names and what they are more consistent.

For all who are new to events, they can tell you a lot about query and system performance, specially scenarios like .. why a query isn’t making progress ? – One example of a wait event may be IO / DataFileRead, which points that a process is currently waiting for data to be read from disk or the page cache.

So, talking about those waits, first one is event ‘RecoveryWalAll‘ which tells recovery is waiting for WAL from a stream, that could be from any kind of source i.e. local, archive or stream, is now changed to ‘RecoveryWalStream’.

Another one is ‘RecoveryWalStream‘ which can help you to make sure that each standby does not have more WAL to replay or helps to identify what is waiting for WAL from a stream at recovery, is now changed/renamed to RecoveryRetrieveRetryInterval. I have also noticed a change in event class or category of RecoveryRetrieveRetryInterval to Timeout from Activity.

Hope It Helps
Prashant Dixit

Posted in Advanced, troubleshooting | Tagged: , , | Leave a Comment »

Quarantine a SQL ? This can save from a system performance pandemic …

Posted by FatDBA on July 6, 2021

Hi Guys,

Last weekend I was asked to examine a performance problem where customer was observing sporadic resource spikes on CPU & PIOs (Physical IO scans), and had no idea how to handle the situation. They already have identified the problematic SQL and asked me to take a look, while doing initial investigation on the system, I found the resource manager was enabled and was killing the ill SQL every time when it tries to breach the limits set on CPU & other resources.

I spent some time understanding query behavior and performance, and I found this a scenario of SQL plan (PHV) flip where the query optimizer toggles between a good and a worst plan. This being a time sensitive issue and customer needs a quick fix before I identify the reason behind the PHV flip, we have to think of any technique on how we can stop the SQL execution with bad PHVs. This was an Oracle EE 19c system running on Exadata, I immediately proposed for a solution using new feature of ‘SQL Quarantine‘, which helps to prevent reuse of same execution plan which was terminated by resource managers due to resource limits.

Let me show you how to do that, here I will demonstrate how to quarantine a SQL & all its execution plans.

Well this is just a use case, you can also quarantine the SQL for all its PHVs or execution plans or even through the SQL Text. Here I will show how you can quarantine a SQL for all its execution plans.

Note: This feature is currently available only on Enterprise Edition on Engineered Systems, like Exadata and Exadata Cloud Service, but for this demo, I will be enabling the ‘_exadata_feature_on‘ parameter for demo purposes on my sandbox setup.

** Please don’t touch this parameter in your production environments if you’re not running on Oracle EE on the top of Exadata or Exadata Cloud services.

[oracle@canttowinsec ~]$ !sql
sqlplus / as sysdba

SQL*Plus: Release 19.0.0.0.0 - Production on Sat Jul 3 23:33:41 2021
Version 19.3.0.0.0

Copyright (c) 1982, 2019, Oracle.  All rights reserved.


SQL>
SQL> @hidden
Enter value for param: exadata_feature_on
old   5:    and a.ksppinm like '%&param%'
new   5:    and a.ksppinm like '%exadata_feature_on%'

Parameter                                     Session Value             Instance Value            descr
--------------------------------------------- ------------------------- ------------------------- ------------------------------------------------------------
_exadata_feature_on                           FALSE                     FALSE                     Exadata Feature On

SQL>
SQL>


SQL> shut immediate
Database closed.
Database dismounted.
ORACLE instance shut down.
SQL>
SQL>


SQL> startup
ORACLE instance started.

Total System Global Area  922745216 bytes
Fixed Size                  8903040 bytes
Variable Size             843055104 bytes
Database Buffers           67108864 bytes
Redo Buffers                3678208 bytes
Database mounted.
Database opened.
SQL>
SQL>
SQL>



SQL>
SQL> @hidden
Enter value for param: exadata_feature_on
old   5:    and a.ksppinm like '%&param%'
new   5:    and a.ksppinm like '%exadata_feature_on%'

Parameter                                     Session Value             Instance Value            descr
--------------------------------------------- ------------------------- ------------------------- ------------------------------------------------------------
_exadata_feature_on                           TRUE                      TRUE                      Exadata Feature On

SQL>




-- For this demo I have created one table with 1000000 rows. 

SQL> select count(*) from bigtab;

  COUNT(*)
----------
   1000000

Next check the SQLID, PHV and TEXT of the SQL for which you want to setup this QUARANTINE feature and will create the quarantine configuration for it. You can set it for SQL ID, SQL ID + PHV or for SQL TEXT.

SQL> select sql_id, plan_hash_value, sql_text from v$sqlarea where sql_text like '%select * from bigtab%';

SQL_ID        PLAN_HASH_VALUE
------------- ---------------
SQL_TEXT
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
6fwqzurbc8y7k       441133017
select * from bigtab

SQL>


-- You can create CREATE_QUARANTINE BY SQL_ID 
quarantine_config := DBMS_SQLQ.CREATE_QUARANTINE_BY_SQL_ID(SQL_ID => '&sql_id')
quarantine_config := DBMS_SQLQ.CREATE_QUARANTINE_BY_SQL_ID(SQL_ID => '&sql_id', PLAN_HASH_VALUE => '&PLAN_HASH_VALUE');

-- You can create CREATE_QUARANTINE_BY_SQL_TEXT.
quarantine_config := DBMS_SQLQ.CREATE_QUARANTINE_BY_SQL_TEXt(SQL_TEXT => to_clob('select count(*) from emp'));



-- This command creates a quarantine configuration for SQL ID '6fwqzurbc8y7k' and all its execution plans.
SQL>
SQL> DECLARE
quarantine_sql VARCHAR2(30);
BEGIN
quarantine_sql :=   DBMS_SQLQ.CREATE_QUARANTINE_BY_SQL_ID(SQL_ID => '6fwqzurbc8y7k');
END;
/  

PL/SQL procedure successfully completed.

SQL>



-- Check if quarantine config/profile is created.

SQL> select NAME, SQL_TEXT, ELAPSED_TIME, cpu_time,CREATED, ENABLED from dba_sql_quarantine;


NAME                           SQL_TEXT                       ELAPSED_TIME         CPU_TIME                       CREATED                        ENA
------------------------------ ------------------------------ -------------------- ------------------------------ ------------------------------ ---
SQL_QUARANTINE_2akx3bq44wvc5   select * from bigtab           ALWAYS               ALWAYS                         04-JUL-21 12.09.25.567422 AM   YES


Now next we will use the DBMS_SQLQ.ALTER_QUARANTINE procedure to put limits on resource usage i.e. Elapsed time, CPU, IO (MBs), Number of physical (PIOs) requests, Number of logical (LIOs) requests.

SQL>
SQL> BEGIN
    DBMS_SQLQ.ALTER_QUARANTINE(
       QUARANTINE_NAME => 'SQL_QUARANTINE_2akx3bq44wvc5',
       PARAMETER_NAME  => 'CPU_TIME',
       PARAMETER_VALUE => '5');

    DBMS_SQLQ.ALTER_QUARANTINE(
       QUARANTINE_NAME => 'SQL_QUARANTINE_2akx3bq44wvc5',
       PARAMETER_NAME  => 'ELAPSED_TIME',
       PARAMETER_VALUE => '10');
END;
/  2    3    4    5    6    7    8    9   10   11   12

PL/SQL procedure successfully completed.

SQL>


-- let's verify limits that we have set against this SQL Quarantine profile.

NAME                           SQL_TEXT                       ELAPSED_TIME         CPU_TIME                       CREATED                        ENA
------------------------------ ------------------------------ -------------------- ------------------------------ ------------------------------ ---
SQL_QUARANTINE_2akx3bq44wvc5   select * from bigtab           10                   5                              04-JUL-21 12.12.41.918609 AM   YES

Next we will setup the RESOURCE MANAGER.

-- Create a pending area.
SQL> begin
 dbms_resource_manager.create_pending_area();
 end;
 /  2    3    4

PL/SQL procedure successfully completed.

-- Create a consumer group with name 'DIXITPOC_GROUP'.
SQL> begin
 dbms_resource_manager.create_consumer_group(CONSUMER_GROUP=>'DIXITPOC_GROUP',COMMENT=>'To test SQL quarantine feature for one SQL');
 end;
 /  2    3    4

PL/SQL procedure successfully completed.


-- Now bind the consumer group with the user 'DIXIT', I mean this could be any group through which you will execute the query.
SQL> begin
 dbms_resource_manager.set_consumer_group_mapping(attribute => 'ORACLE_USER',value => 'DIXIT',consumer_group =>'DIXITPOC_GROUP' );
 end;
  /  2    3    4

PL/SQL procedure successfully completed.


-- Create a resource plan with name 'POC_FOR_QUARANTINE'.
SQL> begin
dbms_resource_manager.create_plan(plan => 'POC_FOR_QUARANTINE',comment => 'Kill SQL and its other execution plans when exceeding CPU and Elapsed seconds');
end;
/  2    3    4

PL/SQL procedure successfully completed.


-- Create a plan directive by allocating resource plan 'POC_FOR_QUARANTINE' to consumer group 'DIXITPOC_GROUP'. I am setting execution limit of 5 seconds for the SQL.
SQL> begin
 dbms_resource_manager.create_plan_directive(
 plan => 'POC_FOR_QUARANTINE',
 group_or_subplan => 'DIXITPOC_GROUP',
 comment => 'Kill SQL and its other execution plans when exceeding CPU and Elapsed seconds',
 switch_group=>'CANCEL_SQL',
 switch_time => 5,
 switch_estimate=>false);
 end;
 /  2    3    4    5    6    7    8    9   10

PL/SQL procedure successfully completed.




-- Allocate full resources to rest of the sessions which are not part of this plan.
SQL> begin
 dbms_resource_manager.create_plan_directive(PLAN=> 'POC_FOR_QUARANTINE', GROUP_OR_SUBPLAN=>'OTHER_GROUPS',COMMENT=>'leave rest as is');
 end;
 /  2    3    4

PL/SQL procedure successfully completed.



-- Validate and submit pending area.
SQL> begin
 dbms_resource_manager.validate_pending_area();
 end;
 /  2    3    4

PL/SQL procedure successfully completed.

SQL> begin
 dbms_resource_manager.submit_pending_area();
 end;
 /  2    3    4

PL/SQL procedure successfully completed.

SQL>


-- Grant switch privilege to the DIXIT user which will switch it to 'DIXITPOC_GROUP' group.
SQL> begin
dbms_resource_manager_privs.grant_switch_consumer_group('DIXIT','DIXITPOC_GROUP',false);
end;
/  2    3    4

PL/SQL procedure successfully completed.

-- Create initial consumer group for DIXIT user for 'DIXITPOC_GROUP' group.
SQL> begin
dbms_resource_manager.set_initial_consumer_group('DIXIT','DIXITPOC_GROUP');
end;
/  2    3    4

PL/SQL procedure successfully completed.

SQL>

Next set the resource manager at the database level, we will have to set the RESOURCE_MANAGER_PLAN to value or name of the plan ‘POC_FOR_QUARANTINE’.

SQL> show parameter resource

NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
resource_limit                       boolean     TRUE
resource_manage_goldengate           boolean     FALSE
resource_manager_cpu_allocation      integer     5
resource_manager_plan                string
SQL> 


SQL> alter DIXITtem set RESOURCE_MANAGER_PLAN = 'POC_FOR_QUARANTINE';
DIXITtem altered.

SQL> show parameter resource

NAME                                 TYPE        VALUE
------------------------------------ ----------- ------------------------------
resource_limit                       boolean     TRUE
resource_manage_goldengate           boolean     FALSE
resource_manager_cpu_allocation      integer     5
resource_manager_plan                string      POC_FOR_QUARANTINE
SQL>

Execute the SQL for the first time, you will get ‘active time limit exceeded’ due to RM.

SQL>
SQL> select * from emp;

ERROR at line 2:
ORA-00040: active time limit exceeded - call aborted

Now, if you run the SQL for the second time, you will see your quarantine plan/profile will be used and will abort the execution of the SQL.

SQL>
SQL> select * from emp;

ERROR at line 2:
ORA-56955: quarantined plan used

Hope It Helped
Prashant Dixit

Posted in Advanced, troubleshooting | Tagged: , , , , | Leave a Comment »

Migration assessment report, possible with ora2pg ?

Posted by FatDBA on July 3, 2021

Hi Guys,

Last week someone asked if me if it’s possible to generate the migration assessment report (of any sort) if using ora2pg for migration ? – The entire team was coming from EDB PostgreSQL migration where they used their migration toolkit, and there you can generate something similar, but with their current project, ora2pg was finalized as a migration tool for Oracle to vanilla PostgreSQL migration.

And the answer to their question was YES, it’s possible to generate the migration assessment report with ora2pg too, which is very useful as it performs an assessment for the potential migration problems or difficulties. It allocates a cost per object types and also provides other details like invalid object counts, total number of object types, comments and more description about objects etc.

It inspects all database objects , functions, stored procedures to detect if there are some objects or PL/SQL code that cannot be automatically converted by the tool. At the end of the report it calculates the overall cost (sum of all costs) and assigns an approximate number of days or time to perform the migration.

Default time per “cost evaluation unit” is 5 minutes, and outputs can be generated in text (default), html or csv formats.

Let’s generate a sample report, but first let me show you the version of ora2pg, and next will call the report for Oracle database running on 18c for a particular schema.

$
$ ora2pg -t SHOW_VERSION -c config/ora2pg.conf
Oracle Database 18c Enterprise Edition Release 18.0.0.0.0


-- Here we used 'estimate_cost' is to activate the migration cost evaluation with SHOW_REPORT

$ ora2pg -t show_report  --estimate_cost -c config/ora2pg.conf --dump_as_html > /tmp/orapgtestmigr/ora2pgreport.html

-------------------------------------------------------------------------------
Ora2Pg v20.0 - Database Migration Report
-------------------------------------------------------------------------------
Version  Oracle Database 18c Enterprise Edition Release 18.0.0.0.0
Schema   DIXITSCH
Size     88.19 MB

Below is the screen snip of the report head.

Please click below link to view the full report.

Hope It Helped!
Prashant Dixit

Posted in Advanced | Tagged: , , , | Leave a Comment »

Single command to prepare a DB for a Data Guard configuration in Oracle 21c – PREPARE DATABASE FOR DATA GUARD WITH

Posted by FatDBA on June 28, 2021

Hi Guys,

Another cool feature I have identified while doing a POC on Oracle 21c database!
Starting with Oracle 21c, now you can create Primary database in a Data Guard Broker configuration using one single command PREPARE DATABASE FOR DATA GUARD WITH <options>. This command will do all the work for you i.e. database preparation, enables force logging, set required parameters, creates standby redo log files, enables Flashback & enables Archivelog.

Below command will prepare the database with name dixitdb to use as a primary database in DG broker configuration.

[oracle@canttowin ~]$ dgmgrl
DGMGRL for Linux: Release 21.0.0.0.0 - Production on Sat Jun 26 11:10:11 2021
Version 21.1.0.0.0

Copyright (c) 1982, 2020, Oracle and/or its affiliates.  All rights reserved.

Welcome to DGMGRL, type "help" for information.
DGMGRL>

DGMGRL> PREPARE DATABASE FOR DATA GUARD WITH DB_UNIQUE_NAME IS dixitdb;
DB_RECOVERY_FILE_DEST IS "$ORACLE_BASE_HOME/dbs/"
DB_RECOVERY_FILE_DEST_SIZE is "400G"
DG_BROKER_CONFIG_FILE1 IS "$ORACLE_HOME/dbs/testfile.dat"
DG_BROKER_CONFIG_FILE2 IS "$ORACLE_HOME/dbs/testfile2.dat";
Preparing database "dixitdb" for Data Guard.
Creating server parameter file (SPFILE) from initialization parameter memory values.
Database must be restarted after creating the server parameter (SPFILE).
Shutting down database "dixitdb".
Database closed.
Database dismounted.
ORACLE instance shut down. Starting database "dixitdb" to mounted mode.
ORACLE instance started.
Database mounted. Server parameter file (SPFILE) is "ORACLE_BASE_HOME/dbs/spdixitdb.ora".
Initialization parameter DB_UNIQUE_NAME set to 'dixitdb'.
Initialization parameter DB_FILES set to 1024.
Initialization parameter LOG_BUFFER set to 401355151.
Primary database must be restarted after setting static initialization parameters. Primary database must be restarted to enable archivelog mode.
Shutting down database "dixitdb".
Database dismounted.
ORACLE instance shut down.
Starting database "dixitdb" to mounted mode.
ORACLE instance started.
Database mounted.
Initialization parameter DB_FLASHBACK_RETENTION_TARGET set to 120.
Initialization parameter DB_BLOCK_CHECKSUM set to 'TYPICAL'.
Initialization parameter DB_LOST_WRITE_PROTECT set to 'TYPICAL'.
Initialization parameter PARALLEL_THREADS_PER_CPU set to 1.
Removing RMAN archivelog deletion policy 1.
Removing RMAN archivelog deletion policy 2.
RMAN configuration archivelog deletion policy set to SHIPPED TO ALL STANDBY.
Initialization parameter DB_RECOVERY_FILE_DEST_SIZE set to '200G'.
Initialization parameter DB_RECOVERY_FILE_DEST set to 'ORACLE_BASE_HOME/dbs/'. Initialization parameter DG_BROKER_START set to FALSE.
Initialization parameter DG_BROKER_CONFIG_FILE1 set to 'ORACLE_HOME/dbs/testfile.dat'.
Initialization parameter DG_BROKER_CONFIG_FILE2 set to 'ORACLE_HOME/dbs/testfile2.dat'.
LOG_ARCHIVE_DEST_n initialization parameter already set for local archival.
Initialization parameter LOG_ARCHIVE_DEST_2 set to 'location=use_db_recovery_file_dest valid_for=(all_logfiles, all_roles)'.
Initialization parameter LOG_ARCHIVE_DEST_STATE_2 set to 'Enable'.
Initialization parameter STANDBY_FILE_MANAGEMENT set to 'MANUAL'.
Standby log group 4 will be dropped because it was not configured correctly.
Standby log group 3 will be dropped because it was not configured correctly.
Adding standby log group size 26214400 and assigning it to thread 1.
Initialization parameter STANDBY_FILE_MANAGEMENT set to 'AUTO'.
Initialization parameter DG_BROKER_START set to TRUE.
Database set to FORCE LOGGING. Database set to ARCHIVELOG.
Database set to FLASHBACK ON.
Database opened.


Hope It Helped!
Prashant Dixit

Posted in Advanced | Tagged: , , | Leave a Comment »

Getting SQL Net message from dblink, have you tried DRIVING_SITE hint ?

Posted by FatDBA on June 27, 2021

Hi Guys,

Last week I was asked to take a look in to one performance problem where customer DBA reported excessive ‘SQL*Net message from dblink’ in the system. As I don’t have direct access to the box, I asked him to share both AWR and ASH performance reports for the time when they observe the problem. And yes, he was right, there were few waits on this event but with very high average elapsed times or single execution times. Next I verified the ASH data for the period and I was able to identify this SQL (SELECT) which is frequently waiting on this wait tvent.

About this event, as you guys know SQL Net messages are classified as Network classed events and happens when some part of the final dataset is coming from a remote database which is accessed via DBLink. Alright, so we know the problem and we have the problematic SQL, let’s simulate the problem and the solution.

I will first create the DBLink that I will be using to access the remote data which is present in a different database on a different server/host and next I will create the sample table with some test data both on Site A and Site B.

Reference used:
BIGTAB: Table on Site 1, local site.
BIGTAB2: Table on site 2, remote location

SQL>
SQL> CREATE DATABASE LINK dixitdlink1
  2     CONNECT TO dixit IDENTIFIED BY oracle90
   USING '(DESCRIPTION=
            (ADDRESS = (PROTOCOL = TCP)(HOST = canttowinsec.quebecdomain)(PORT = 1521))
            (CONNECT_DATA=(SERVICE_NAME=dhavaldb.quebecdomain))
          )';  

Database link created.

SQL>


-- On source database, on local host.
CREATE TABLE BIGTAB (
   id         NUMBER,
   weight     NUMBER,
   adate      DATE
);

Table created.

INSERT INTO BIGTAB (id, weight, adate)
SELECT MOD(ROWNUM,1000),
       DBMS_RANDOM.RANDOM,
       SYSDATE-1000+DBMS_RANDOM.VALUE(0,1000)
 FROM all_objects
/

67447 rows created.


SQL> delete from prashant.BIGTAB where rownum < 60000;

59999 rows deleted.




-- This I have created on target database, on a remote host.
CREATE TABLE BIGTAB2 (
   id         NUMBER,
   weight     NUMBER,
   adate      DATE
);

Table created.


INSERT INTO BIGTAB2 (id, weight, adate)
SELECT MOD(ROWNUM,1000),
       DBMS_RANDOM.RANDOM,
       SYSDATE-1000+DBMS_RANDOM.VALUE(0,1000)
 FROM all_objects
/

67447 rows created.

Alright, the test data is created on both the sites, and in order to mimic the exact case that I’d faced, I have intentionally deleted 59999 table from table that exists in site 1. So now we have table BIGTAB with 7735 records and on remote database we have a table with name BIGTAB2 with 67447 rows. Let’s execute few queries and do a Inner Join on both the tables.

Test 1: With no hint or tuning/tweaking of any kind. Asking to bring all data from remote site here to local site and then perform the join operation or this can also be done if we put the hint and mention name of the local table, or table that is small i.e. /*+ DRIVING_SITE(BIGTAB) */

SQL> select * from prashant.BIGTAB, dixit.BIGTAB2@dixitdlink1 where ADATE=DATEOFREGISTER;

        ID     WEIGHT ADATE             ID     WEIGHT DATEOFREG
---------- ---------- --------- ---------- ---------- ---------
       502 -1.363E+09 09-MAR-20        989 1353403341 09-MAR-20
       577 -185863752 22-FEB-21        247 -626839962 22-FEB-21
........
............
...............
.................


Execution Plan
----------------------------------------------------------
Plan hash value: 2705338834

----------------------------------------------------------------------------------------------
| Id  | Operation          | Name    | Rows  | Bytes | Cost (%CPU)| Time     | Inst   |IN-OUT|
----------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |         | 19931 |  1362K|   144   (0)| 00:00:01 |        |      |
|*  1 |  HASH JOIN         |         | 19931 |  1362K|   144   (0)| 00:00:01 |        |      |
|   2 |   TABLE ACCESS FULL| BIGTAB  |  6948 |   237K|    68   (0)| 00:00:01 |        |      |
|   3 |   REMOTE           | BIGTAB2 | 19930 |   681K|    76   (0)| 00:00:01 | DIXIT~ | R->S |
----------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   1 - access("ADATE"="DATEOFREGISTER")

Remote SQL Information (identified by operation id):
----------------------------------------------------

   3 - SELECT "ID","WEIGHT","DATEOFREGISTER" FROM "DIXIT"."BIGTAB2" "BIGTAB2"
       (accessing 'DIXITDLINK1.ONTADOMAIN' )


Note
-----
   - dynamic statistics used: dynamic sampling (level=2)

So, this was happening there on that day, on that system with that query waiting on ‘SQL*Net message from dblink‘, now this is the right time I should introduce the DRIVING_SITE hint, I have been trying and experimenting with this hint for a long time now, maybe since my 9i days.

This hint is useful for distributed queries to force optimizer to use a particular site as a driving site, or it instructs the optimizer to execute the query at a different site than that selected by the database. Like in previous test case all rows from remote host which matches the condition are sent to the local site and the join is finally executed on the local site.

Now, I am going to use the DRIVING_SITE (will mention name of the remote table) and force the optimizer to perform all join operations at the remote site, hence the query will be executed there and the result set is returned to the local site.

So, you should try with the DRIVING_SITE hint on the site where the huge table resides!

Test 2: With DRIVING_HINT on remote table BIGTAB2 which is big and has 67447 records.

SQL> select /*+ DRIVING_SITE(BIGTAB2) */ * from prashant.BIGTAB, dixit.BIGTAB2@dixitdlink1 where ADATE=DATEOFREGISTER;

        ID     WEIGHT ADATE             ID     WEIGHT DATEOFREG
---------- ---------- --------- ---------- ---------- ---------
       502 -1.363E+09 09-MAR-20        989 1353403341 09-MAR-20
       577 -185863752 22-FEB-21        247 -626839962 22-FEB-21
.....
.......
........
..........



Execution Plan
----------------------------------------------------------
Plan hash value: 2214131741

--------------------------------------------------------------------------------------------------
| Id  | Operation              | Name    | Rows  | Bytes | Cost (%CPU)| Time     | Inst   |IN-OUT|
--------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT REMOTE|         |    82 |  5740 |    70   (0)| 00:00:01 |        |      |
|*  1 |  HASH JOIN             |         |    82 |  5740 |    70   (0)| 00:00:01 |        |      |
|   2 |   REMOTE               | BIGTAB  |    82 |  2870 |     2   (0)| 00:00:01 |      ! | R->S |
|   3 |   TABLE ACCESS FULL    | BIGTAB2 | 53126 |  1815K|    68   (0)| 00:00:01 | DHAVA~ |      |
--------------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   1 - access("A2"."ADATE"="A1"."DATEOFREGISTER")

Remote SQL Information (identified by operation id):
----------------------------------------------------

   2 - SELECT "ID","WEIGHT","ADATE" FROM "PRASHANT"."BIGTAB" "A2" (accessing '!' )


Note
-----
   - fully remote statement
   - dynamic statistics used: dynamic sampling (level=2)

Look at the cost, rows processed and bytes (data processed), all of them are reduced by a great extent. Of course not much elapsed time difference you see here in this example as the dataset is small and is a less complex query, you can try it yourself and see the magic!

Hope It Helped!
Prashant Dixit

Posted in Advanced, troubleshooting | Tagged: , , | Leave a Comment »

Running on Oracle 19c ? – Why to enable full supplemental logging when SUBSET DATABASE REPLICATION is there

Posted by FatDBA on June 26, 2021

Hi Folks,

Today’s post is about one of the cool feature added in Oracle 19c, that is very useful and I recently discovered about it while working on one GG setup.

This is about Supplemental logging which enables database to capture extra logging for a redo-based application i.e. logical standby or Golden Gate, this may require that additional columns be logged in the redo log files and causes a great stress and overhead on the database. Things looks very unworthy specially if you are doing only a partial Golden Gate replication, I mean for only few tables.

Starting from Oracle 19c, this is something that you can now control. With this version, you can enable the fine grain supplemental logging (SUBSET DATABASE REPLICATION) which will reduce lot of logging overhead. This feature allows you to disable supplemental logging on the entire database and on all tables. So, if you are running on Oracle 19c and your have the COMPATIBLE parameter to 19.0.0.0.0 or greater and have enabled enable_goldengate_replication parameter to TRUE, you can use this option.

Note: I know few of the products where ‘Supplemental Log Data Subset Database Replication’ doesn’t work or the product doesn’t support it, i.e. HVR 6

SQL> select BANNER_FULL from v$version;

BANNER_FULL
--------------------------------------------------------------------------------
Oracle Database 19c Enterprise Edition Release 19.0.0.0.0 - Production
Version 19.3.0.0.0


SQL> SELECT  FORCE_LOGGING, supplemental_log_data_min, supplemental_log_data_sr from v$database;

FORCE_LOGGING                           SUPPLEMENTAL_LOG_DATA_MIN                                    SUPPLEMENTAL_LOG_DATA_SR
--------------------------------------- ------------------------------------------------------------ ----------------------------------------
YES                                     YES                                                          NO


SQL> ALTER DATABASE ADD SUPPLEMENTAL LOG DATA SUBSET DATABASE REPLICATION;

Database altered.

SQL> SELECT  FORCE_LOGGING, supplemental_log_data_min, supplemental_log_data_sr from v$database;

FORCE_LOGGING                           SUPPLEMENTAL_LOG_DATA_MIN                                    SUPPLEMENTAL_LOG_DATA_SR
--------------------------------------- ------------------------------------------------------------ ----------------------------------------
YES                                     YES                                                          YES

You can disable it anytime using below mentioned steps/commands.

SQL>
SQL> ALTER DATABASE ADD SUPPLEMENTAL LOG DATA;

Database altered.

SQL>  ALTER DATABASE DROP SUPPLEMENTAL LOG DATA SUBSET DATABASE REPLICATION;

Database altered.

SQL> SELECT  FORCE_LOGGING, supplemental_log_data_min, supplemental_log_data_sr from v$database;

FORCE_LOGGING                           SUPPLEMENTAL_LOG_DATA_MIN                                    SUPPLEMENTAL_LOG_DATA_SR
--------------------------------------- ------------------------------------------------------------ ----------------------------------------
YES                                     YES                                                          NO

Hope It Helps!
Prashant Dixit

Posted in Advanced | Tagged: , , | 2 Comments »

Data replication between Oracle & PostgreSQL using Golden Gate.

Posted by FatDBA on June 24, 2021

Hi Mates,

Recently I was doing one migration where Golden Gate was used for data movement/migration between this source which is was Oracle 19.1 database to target, that was PostgreSQL 12. This was the first time I was doing this for PostgreSQL, I earlier tried with MySQL, Teradata and Cassandra and they were little tricky, specially the one with Cassandra. Let’s see how this one with PostgreSQL goes …

Okay, so this post is all about setting a test replication setup between Oracle and PostgreSQL database. Without further ado, let’s get started!

Details:
Source: Oracle 19c (19.3.0) database
Target: Vanilla/Open-Source PostgreSQL 12.6.7
Golden Gate Version: Golden Gate 19.1 for Oracle (Source), Golden Gate 19.1 for PostgreSQL Database (Target Host).
Source Hostname: canttowinsec.quebecdomain (IP 192.168.154.129)
Target Hostname: canttowin.ontadomain (192.168.154.128)

ON SOURCE:

Let’s first create the user and one test table on the source database which we will use to do this demo.

sqlplus / as sysdba

SQL*Plus: Release 19.0.0.0.0 - Production on Wed Jun 23 15:21:01 2021
Version 19.3.0.0.0
Copyright (c) 1982, 2019, Oracle.  All rights reserved.
Connected to:
Oracle Database 19c Enterprise Edition Release 19.0.0.0.0 - Production Version 19.3.0.0.0

SQL> create user opeth identified by opeth;

User created.

SQL> grant connect to opeth;

Grant succeeded.

SQL> conn opeth/opeth
Connected.

SQL>  create table testtable (col1 number, col2 varchar2(20));

Table created.

SQL> alter table testtable add primary key (col1);

Table altered.

Next I will install Golden Gate using a response file (silent method to install).

[oracle@canttowin Disk1]$ ./runInstaller -silent -nowait -responseFile /home/oracle/ggdirpsql/Disk1/response/oggcore.rsp
Starting Oracle Universal Installer...

Checking Temp space: must be greater than 120 MB.   Actual 6549 MB    Passed
Checking swap space: must be greater than 150 MB.   Actual 3945 MB    Passed
Preparing to launch Oracle Universal Installer from /tmp/OraInstall2021-06-18_04-33-50AM. Please wait ...[oracle@canttowin Disk1]$ You can find the log of this install session at:
 /u01/app/oraInventory/logs/installActions2021-06-18_04-33-50AM.log
The installation of Oracle GoldenGate Core was successful.
Please check '/u01/app/oraInventory/logs/silentInstall2021-06-18_04-33-50AM.log' for more details.
Successfully Setup Software.

Next, I will set the environmental variables to avoid any errors while calling the GG cli and will create the manager process.

[oracle@canttowinsec ggdirpsql]$ pwd
/home/oracle/ggdirpsql
[oracle@canttowinsec ggdirpsql]$ export PATH=$PATH:/home/oracle/ggdirpsql
[oracle@canttowinsec ggdirpsql]$ export LD_LIBRARY_PATH=$ORACLE_HOME/lib:/home/oracle/ggdirpsql
[oracle@canttowinsec ggdirpsql]$


[oracle@canttowin ggdir]$ ./ggsci

Oracle GoldenGate Command Interpreter for Oracle
Version 19.1.0.0.4 OGGCORE_19.1.0.0.0_PLATFORMS_191017.1054_FBO
Linux, x64, 64bit (optimized), Oracle 19c on Oct 17 2019 21:16:29
Operating system character set identified as UTF-8.

Copyright (C) 1995, 2019, Oracle and/or its affiliates. All rights reserved.


GGSCI (canttowinsec.quebecdomain) 7> view param mgr

PORT 7810


GGSCI (canttowinsec.quebecdomain) 1> start mgr
Manager started


GGSCI (canttowinsec.quebecdomain) 1> info all

Program     Status      Group       Lag at Chkpt  Time Since Chkpt

MANAGER     RUNNING

Now when manager process is running, next will login to the source database using Golden Gate and will list the table what we have created in the beginning, this is just to assure if everything is okay at the source.

GGSCI (canttowinsec.quebecdomain) 6> dblogin userid opeth, password opeth
Successfully logged into database.


GGSCI (canttowinsec.quebecdomain as opeth@dhavaldb) 10> list tables opeth.*
opeth.testtable

Found 1 tables matching list criteria.


All good, lets's create the EXTRACT process on the source system.

GGSCI (canttowinsec.quebecdomain) 2> edit param EORPSQL

GGSCI (canttowinsec.quebecdomain) 1> view params EORPSQL

EXTRACT EORPSQL
USERID opeth, password opeth
RMTHOST 192.168.154.128, MGRPORT 7810
RMTTRAIL ./dirdat/ep
TABLE opeth.testtable;


GGSCI (canttowinsec.quebecdomain) 4> add extract EORPSQL, tranlog, begin now
EXTRACT added.


GGSCI (canttowinsec.quebecdomain) 5> add exttrail ./dirdat/ep, extract EORPSQL, megabytes 5
EXTTRAIL added.

GGSCI (canttowinsec.quebecdomain) 6> start EORPSQL

Sending START request to MANAGER ...
EXTRACT EORPSQL starting



GGSCI (canttowinsec.quebecdomain) 11> info EORPSQL

EXTRACT    EORPSQL   Last Started 2021-06-23 15:10   Status RUNNING
Checkpoint Lag       00:00:00 (updated 00:00:07 ago)
Process ID           10714
Log Read Checkpoint  Oracle Redo Logs
                     2021-06-23 15:11:11  Seqno 15, RBA 31941120
                     SCN 0.2711866 (2711866)

ON TARGET:

Now we are done with all prerequisites, installation and configurations at the source end, let’s move to the target system now where we have our PostgreSQL database running. I will create the same table what we have created on Oracle database (source).

[postgres@canttowin ggdirpsql]$ psql -p 5432
psql (12.6.7)
Type "help" for help.

postgres=# \c opeth opeth
You are now connected to database "opeth" as user "opeth".

opeth=# CREATE TABLE "public"."testtable"
opeth-# (
opeth(#   "col1" integer NOT NULL,
opeth(#   "col2" varchar(20),
opeth(#   CONSTRAINT "PK_Col111" PRIMARY KEY ("col1")
opeth(# )
opeth-# ;
CREATE TABLE
opeth=#
opeth=#
opeth=# \dt+
                     List of relations
 Schema |  Name  | Type  |  Owner   |  Size   | Description
--------+--------+-------+----------+---------+-------------
 public | testtable | table | postgres | 0 bytes |
(1 row)

Next comes the most important part, that is to create the odbc.ini file, Golden Gate uses an ODBC connection to connect to the Postgres database. The ODBC driver is shipped with the installation and on Unix you have to create the ODBC configuration file which is commonly called odbc.ini on your own. You need to create this file in GG_HOME directory on the target system.

[postgres@canttowin ggdirpsql]$ more odbc.ini
[ODBC Data Sources]
PostgreSQL on pgsql
[ODBC]
IANAAppCodePage=4
InstallDir=/home/postgres/ggdirpsql
[pg12db]
Driver=/home/postgres/ggdirpsql/lib/GGpsql25.so
Description=Postgres driver
Database=opeth
HostName=canttowin.ontadomain
PortNumber=5432
LogonID=opeth
Password=opeth



Reference:
Driver=<your goldengate home directory>/lib/GGpsql25.so
InstallDir= <where you have installed your GG software on target server.
IANAAppCodePage= <The value 4 represents the ISO-8859-1 character set>
Database=<Postgres Database Name>
HostName=<Hostname of the Postgres database>
PortNumber=<Port number of the Postgres database>
LogonID=<Username of the Postgres database>
Password=<Password of the Postgres database>

Now, let’s set the environmental variables to point locations for odbc file along with lib directory and installation dir and create all required GG specific directories on the target database server.

[postgres@canttowin ggdirpsql]$ export ODBCINI=/home/postgres/ggdirpsql/odbc.ini
[postgres@canttowin ggdirpsql]$ export PATH=$PATH:/home/postgres/ggdirpsql
[postgres@canttowin ggdirpsql]$  export LD_LIBRARY_PATH=/home/postgres/ggdirpsql/lib


[postgres@canttowin ggdirpsql]$ ./ggsci

Oracle GoldenGate Command Interpreter for PostgreSQL
Version 19.1.0.0.200714 OGGCORE_19.1.0.0.0OGGBP_PLATFORMS_200628.2141
Linux, x64, 64bit (optimized), PostgreSQL on Jun 29 2020 03:59:15
Operating system character set identified as UTF-8.

Copyright (C) 1995, 2019, Oracle and/or its affiliates. All rights reserved.



GGSCI (canttowin.ontadomain) 1> create subdirs

Creating subdirectories under current directory /home/postgres/ggdirpsql

Parameter file                 /home/postgres/ggdirpsql/dirprm: created.
Report file                    /home/postgres/ggdirpsql/dirrpt: created.
Checkpoint file                /home/postgres/ggdirpsql/dirchk: created.
Process status files           /home/postgres/ggdirpsql/dirpcs: created.
SQL script files               /home/postgres/ggdirpsql/dirsql: created.
Database definitions files     /home/postgres/ggdirpsql/dirdef: created.
Extract data files             /home/postgres/ggdirpsql/dirdat: created.
Temporary files                /home/postgres/ggdirpsql/dirtmp: created.
Credential store files         /home/postgres/ggdirpsql/dircrd: created.
Masterkey wallet files         /home/postgres/ggdirpsql/dirwlt: created.
Dump files                     /home/postgres/ggdirpsql/dirdmp: created.




Now, time to create all GG related processes, will start with manager process, followed by replicat 
GGSCI (canttowin.ontadomain) 2> edit param mgr

GGSCI (canttowin.ontadomain) 3> start mgr
Manager started.



GGSCI (canttowin.ontadomain) 2> dblogin sourcedb pg12db userid opeth
Password:

2021-06-23 15:00:58  INFO    OGG-03036  Database character set identified as UTF-8. Locale: en_US.UTF-8.

2021-06-23 15:00:58  INFO    OGG-03037  Session character set identified as UTF-8.
Successfully logged into database.


GGSCI (canttowin.ontadomain as opeth@pg12db) 5> list tables public.*
public.testtable

Found 1 tables matching list criteria.




GGSCI (canttowin.ontadomain as opeth@pg12db) 15> view params RORPSQL

REPLICAT RORPSQL
SOURCEDEFS ./dirdef/testtable.def
SETENV ( PGCLIENTENCODING = "UTF8" )
SETENV (ODBCINI="/home/postgres/ggdirpsql/odbc.ini" )
SETENV (NLS_LANG="AMERICAN_AMERICA.AL32UTF8")
TARGETDB pg12db, USERID opeth, PASSWORD opeth
DISCARDFILE ./dirrpt/diskg.dsc, purge
MAP opeth.testtable, TARGET public.testtable, COLMAP (COL1=col1,COL2=col2);



GGSCI (canttowin.ontadomain as opeth@pg12db) 7> add replicat RORPSQL, NODBCHECKPOINT, exttrail ./dirdat/ep
REPLICAT added.


GGSCI (canttowin.ontadomain as opeth@pg12db) 8> start RORPSQL

Sending START request to MANAGER ...
REPLICAT RORPSQL starting


GGSCI (canttowin.ontadomain as opeth@pg12db) 9> info all

Program     Status      Group       Lag at Chkpt  Time Since Chkpt

MANAGER     RUNNING
REPLICAT    RUNNING     RORPSQL     00:00:59      00:00:03



GGSCI (canttowin.ontadomain as opeth@pg12db) 13>

GGSCI (canttowin.ontadomain as opeth@pg12db) 13> info RORPSQL

REPLICAT   RORPSQL   Last Started 2021-06-23 15:20   Status RUNNING
Checkpoint Lag       00:00:00 (updated 00:00:03 ago)
Process ID           97138
Log Read Checkpoint  File ./dirdat/ep000000000
                     2021-06-23 15:21:41.005117  RBA 29169

All set, we have all processes running at both source and target system. Now, if you running on Golden gate version 12.3 and above, no need to do generate the definition of tables, as the metadata is now present in the trail file itself, like in this demo I am using GG 19.1.

So, only if you running on GG version < 12.3 you need to generate the definition (using defgen file) of the table on source system and move it to the DIRDAT directory on target so that replicat considers that.

Now, let’s do some testing to see if records are flowing from source to target database.

ON SOURCE:

[oracle@canttowinsec dirdef]$ !sql
sqlplus / as sysdba

SQL*Plus: Release 19.0.0.0.0 - Production on Wed Jun 23 15:21:01 2021
Version 19.3.0.0.0
Copyright (c) 1982, 2019, Oracle.  All rights reserved.
Connected to:
Oracle Database 19c Enterprise Edition Release 19.0.0.0.0 - Production Version 19.3.0.0.0

SQL> conn opeth/opeth
Connected.
SQL> insert into testtable values (1,'hello world!');

1 row created.

SQL> select * from testtable;

      COL1 COL2
---------- --------------------
         1 hello world!

SQL>
SQL> commit;

Commit complete.

Let’s see if the same record is reached the target side.

ON TARGET:

[postgres@canttowin ggdirpsql]$ psql -p 5432
psql (12.6.7)
Type "help" for help.

postgres=# \c opeth opeth
You are now connected to database "opeth" as user "opeth".

psql (12.6.7)
Type "help" for help.

opeth=# select * from testtable;
 col1 |  col2
------+---------
   10 | hello world!

(1 rows)

opeth=# 

Cool! it’s pretty easy to setup this solution to migrate your data from Oracle to PostgreSQL database.

Few things to note here
–> You don’t need to install Golden Gate for PostgreSQL, on source system which is where your oracle database is, requires Golden Gate for Oracle.
–> Only on target database is where you have to install Golden Gate for PostgreSQL
–> odbc.ini is the file which you have to create on target end (where your postgresql server is).
–> Always before calling ggsci, set PATH and LD_LIBRARY_PATH, else you might get an error message while calling the cli.
–> Be very careful while creating the odbc.ini file otherwise you will end up with some obscure errors,

Hope It Helped!
Prashant Dixit

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