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Archive for July, 2022

What is a _FIX_CONTROL & DBMS_OPTIM_BUNDLE in Oracle ?

Posted by FatDBA on July 17, 2022

Lately I was in discussion with one of my friend who was facing an issue with Oracle 19c database where the vendor asked him to apply a patch to fix the problem, but he did not want to apply that single patch because their Oracle homes were shared and he didn’t want to increase the complexity of their patching cycles. Then later on Oracle suggested them to try a workaround which requires a setting using fix controls.

So many times Oracle recommends to set a fix control in case of a bug fix, but what exactly are they ? Their purpose ? & tools and methods to control these bug fixes ? This post is all about explaining all of them in detail.

So, What are they ? – Fix controls are bug fix control parameters introduced in 10.2 and they are typically used to enable/disable certain bug fixes in Oracle database. You cannot pull-back any patch, the patch you trying must have the option to use _FIX_CONTROL, and must be visible under V$SYSTEM_FIX_CONTROL views.

Let’s understand this using one of the case where mview push predicate was not happening due to wrong cardinality estimate in one of the production system running on It was rejecting join predicate pushdown (JPPD) transformations and this was avoiding view to be joined with index-based nested-loop join method and causing issues. This was happening all due to bug 21802552. Let’s check if the bug number is present in fix control views and what’s its status.

SQL> select bugno, value, description from v$system_fix_control where bugno=21802552;

---------- ---------- ----------------------------------------------------------------------
  21802552          1 correct cardinality adjusted by DS

-- You can get similar information using DBMS_SQLDIAG.GET_FIX_CONTROL(BUG NUMBER) proc as well.

So, its there in the view’s output and its enabled (value 1), and we can turn it off, lets do it. A proper syntax of using them is given below.

-- To enable:
"_fix_control"='Bugno:ON'    (OR)   "_fix_control"="Bugno:1"

-- To disable:
"_fix_control"='Bugno:OFF'  (OR)   "_fix_control"="Bugno:0"

SQL> ALTER SYSTEM SET "_fix_control" = '21802552:OFF';

System altered.


SQL> select bugno, value, description from v$system_fix_control where bugno=21802552;

---------- ---------- ----------------------------------------------------------------------
  21802552          0 correct cardinality adjusted by DS

-- same was recorded in alert log file as well

ALTER SYSTEM SET _fix_control='21802552:OFF' SCOPE=BOTH;

You can do the same using the new dbms_optim_bundle.set_fix_controls package, it was introduced in to implement Oracle’s approach of ‘Automatic Fix Control Persistence’ framework. Let’s try to the same using said package.

-- This will set given _fix_controls in scope=BOTH on all instances
-- Lets enable it again before we disable it back again
SQL> exec dbms_optim_bundle.set_fix_controls('21802552:1','*', 'BOTH', 'NO');

PL/SQL procedure successfully completed.

SQL> select bugno, value, description from v$system_fix_control where bugno=21802552;

---------- ---------- ----------------------------------------------------------------------
  21802552          1 correct cardinality adjusted by DS

-- Lets roll it back
SQL> exec dbms_optim_bundle.set_fix_controls('21802552:0','*', 'BOTH', 'NO');

PL/SQL procedure successfully completed.

SQL> select bugno, value, description from v$system_fix_control where bugno=21802552;

---------- ---------- ----------------------------------------------------------------------
  21802552          0 correct cardinality adjusted by DS

-- Entry in parameter file made by the dbms_optim_bundle package for fix control
*._fix_control='21802552:0'#added through dbms_optim_bundle package

Hope It Helped!
Prashant Dixit


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How to trace DBMS_STATS and see what is under the hood for a slow sluggish stats collection ..

Posted by FatDBA on July 10, 2022

Recently while working on a slow stats gathering case, someone asked if there is a way to know what’s happening beneath the surface ? what all flags and calculations its doing internally while on the front end the DBMS_STATS still running ? Yes, there is a way! As with most of the Oracle utilities, DBMS_STATS too comes with its own tracing facility which you can call using dbms_stats.set_global_prefs. Today’s post is all about enabling tracing on DBMS_STATS package.

Few of the commonly used DBMS_STATS flags/options are given below, you can always do a combination if wanted to club …

4 = This is to trace table stats
8 = This is to trace index stats
16 = This is to trace columnar stats
512 = auto stats job
1024 = This is to trace parallel executions
4096 = This is to trace partition prunes
16384 = This one traces extended column stats
32768 = To trace approximate NDV (number distinct values) ….

Let’s quickly collect traces for both Indexes and Tables followed by the stas collection for a table and its dependent Indices.

SQL> exec dbms_stats.set_global_prefs('TRACE',4+8);

PL/SQL procedure successfully completed.


SQL> set time on
13:28:55 SQL>
13:28:56 SQL>
13:28:56 SQL>
13:28:56 SQL> exec DBMS_STATS.GATHER_TABLE_STATS (ownname => 'SYS' , tabname => 'BIGTAB',cascade => true, estimate_percent => 10,method_opt=>'for all indexed columns size 1', granularity => 'ALL', degree => 4);

PL/SQL procedure successfully completed.

13:28:59 SQL>
13:29:00 SQL>

SQL> select name, value from v$diag_info where name='Diag Trace';

NAME                                     VALUE
---------------------------------------- ------------------------------------------------------------------------------------
Diag Trace                               /u01/app/oracle/diag/rdbms/localdb/localdb/trace

Alright, we have our trace ready, let’s dig in deep what’s all inside the trace that we have collected for both the Table and it’s Index.
It starts with a standard header in the trace files, followed by all by default explicitly mentioned flags/options with DBMS_STATS in XML format.

-- Header
DBMS_STATS: Record gather table stats operation on table : BIGTAB
DBMS_STATS:   job name: 
DBMS_STATS:    |--> Operation id: 1341
DBMS_STATS: gather_table_stats: <params><param name="block_sample" val="FALSE"/><param name="cascade" val="TRUE"/>
<param name="concurrent" val="FALSE"/><param name="degree" val="4"/><param name="estimate_percent" val="10"/>
<param name="force" val="FALSE"/><param name="granularity" val="ALL"/><param name="method_opt" val="for all indexed columns size 1"/><param name="no_invalidate" val="NULL"/>
<param name="ownname" val="SYS"/><param name="partname" val=""/><param name="reporting_mode" val="FALSE"/><param name="statid" val=""/><param name="statown" val=""/>
<param name="stattab" val=""/><param name="stattype" val="DATA"/><param name="tabname" val="BIGTAB"/></params>
DBMS_STATS: Start gather table stats -- tabname: BIGTAB

Next comes the preferences set for the called Table, ‘BIGTAB’ in our case. This will give you more idea about what all options were set overall and if there is any scope to tune and tweak anyone of them.

DBMS_STATS: Preferences for table SYS.BIGTAB
DBMS_STATS: ================================================================================
DBMS_STATS: SKIP_TIME                                         - 
DBMS_STATS: STATS_RETENTION                                   - 
DBMS_STATS: MON_MODS_ALL_UPD_TIME                             - 
DBMS_STATS: SNAPSHOT_UPD_TIME                                 - 
DBMS_STATS: TRACE                                             - 12
DBMS_STATS: DEBUG                                             - 0
DBMS_STATS: SYS_FLAGS                                         - 1
DBMS_STATS: SPD_RETENTION_WEEKS                               - 53
DBMS_STATS: CASCADE                                           - DBMS_STATS.AUTO_CASCADE
DBMS_STATS: DEGREE                                            - NULL
DBMS_STATS: METHOD_OPT                                        - FOR ALL COLUMNS SIZE AUTO
DBMS_STATS: NO_INVALIDATE                                     - DBMS_STATS.AUTO_INVALIDATE
DBMS_STATS: GRANULARITY                                       - AUTO
DBMS_STATS: PUBLISH                                           - TRUE
DBMS_STATS: STALE_PERCENT                                     - 10
DBMS_STATS: APPROXIMATE_NDV                                   - TRUE
DBMS_STATS: INCREMENTAL                                       - FALSE
DBMS_STATS: AUTOSTATS_TARGET                                  - AUTO
DBMS_STATS: CONCURRENT                                        - OFF
DBMS_STATS: JOB_OVERHEAD_PERC                                 - 1
DBMS_STATS: JOB_OVERHEAD                                      - -1
DBMS_STATS: ENABLE_TOP_FREQ_HISTOGRAMS                        - 3
DBMS_STATS: ENABLE_HYBRID_HISTOGRAMS                          - 3
DBMS_STATS: TABLE_CACHED_BLOCKS                               - 1
DBMS_STATS: INCREMENTAL_LEVEL                                 - PARTITION
DBMS_STATS: OPTIONS                                           - GATHER
DBMS_STATS: GATHER_AUTO                                       - AFTER_LOAD
DBMS_STATS: STAT_CATEGORY                                     - OBJECT_STATS, REALTIME_STATS
DBMS_STATS: SCAN_RATE                                         - 0
DBMS_STATS: GATHER_SCAN_RATE                                  - HADOOP_ONLY
DBMS_STATS: AUTO_STAT_EXTENSIONS                              - OFF
DBMS_STATS: WAIT_TIME_TO_UPDATE_STATS                         - 15
DBMS_STATS: ROOT_TRIGGER_PDB                                  - FALSE
DBMS_STATS: AUTO_TASK_STATUS                                  - OFF
DBMS_STATS: AUTO_TASK_MAX_RUN_TIME                            - 3600
DBMS_STATS: AUTO_TASK_INTERVAL                                - 900
DBMS_STATS: STATS_MODEL_INTERNAL_MINRSQ                       - 0.9
DBMS_STATS: STATS_MODEL                                       - ON
DBMS_STATS: AUTO_STATS_ADVISOR_TASK                           - TRUE

Next sections is where it monitors the stats gathering for Table and its Index, their start and end times were captured, for example : “APPROX_NDV_ALGORITHM => Non-Incremental” was chosen. Table level stats were used i.e. row count, blocks, average row length, sample size, number of indexes etc.

Next it jumps to the Index where it tries to calculate the global statistics for the partitioned Index by aggregating the partition-level statistics but as its a non-partitioned Index, it simples moves to the next stage. Next it triggers the Indexing options (just enables them at the start) i.e. nlb (number of leaf blocks), ndk (number of distinct keys), clf (clustering factor), nblks (number of blocks), sample percentage, degree (parallelism), nrw (key counts) etc by allocating them value 1 which means to consider them.
Finally it starts to analyze the Index structure (In this case it’s not using approximate_ndv algorithm) and calculates the final values of nrw, nlb, ndk, clf etc.

Few Internal options were also used i.e. ctx.conc_ctx, ctx.batching_coeff etc.

-- Table Stats
DBMS_STATS: Started table SYS.BIGTAB. at 28-JUN-22 PM -04:00. Execution phase: SERIAL (NON-CONCURRENT) stattype: DATA Reporting
DBMS_STATS: reporting_man_log_task: target: "SYS"."BIGTAB" objn: 108254 auto_stats: FALSE status: IN PROGRESS ctx.batching_coeff: 0
DBMS_STATS: Start construct analyze using SQL .. Execution Phase: SERIAL (NON-CONCURRENT) granularity: ALL global_requested: NULL pfix: 
DBMS_STATS: APPROX_NDV_ALGORITHM chosen: HLL in incremental (no stats exist)
DBMS_STATS: Start gather part -- ctx.conc_ctx.global_requested: NULL gran: ALL execution phase: 1
DBMS_STATS: APPROX_NDV_ALGORITHM chosen: AS in non-incremental
DBMS_STATS: Start gather_stats.. pfix:  ownname: SYS tabname: BIGTAB pname:  spname:  execution phase: 1
DBMS_STATS: Number of rows in the table = 492190, blocks = , average row length = 19, chain count = , scan rate = 0, sample size = 49219, cstats.count = 3, cind = 2

-- Index part 
DBMS_STATS: Started index SYS.IDX_DIXIT at 28-JUN-22 PM -04:00 granularity: ALL gIdxGran: 
DBMS_STATS: Specified granularity = , New granularity = ALL, Fixed granularity = ALL
DBMS_STATS: granularity ALL
DBMS_STATS: reporting_man_log_task: target: "SYS"."IDX_DIXIT" objn: 108255 auto_stats: FALSE status: IN PROGRESS ctx.batching_coeff: 0
DBMS_STATS:  Gather index subpartition stats...
DBMS_STATS:  Gather index partition stats...
DBMS_STATS:  Gather global index stats...
DBMS_STATS: Start analyze_index_using_sql
DBMS_STATS: tab_stats_stale: BIGTAB not analyzed using full compute
DBMS_STATS:   Number of blocks in the index: 1030
DBMS_STATS: Start execute_analyze_index(owner= SYS, indname= IDX_DIXIT, fobjn= , sample_pct= 10, seed= 0, degree= 4, collect_nrw= 1, collect_nlb= 1, collect_ndk= 1, collect_clf= 1, bmi= 0, iot_ov= 0, iot_sec= 0, ppredtxt= , nblks= 1030)
DBMS_STATS: execute_analyze_index(): Not using approximate_ndv, pct=111.529126213592233009708737864077669903,sample_pct=10
DBMS_STATS: End execute_analyze_index(ssize= 494100, nrw= 494100, nlb= 1027, ndk= 1000, clf= 494100)
DBMS_STATS: target_size: 1027
DBMS_STATS: Finished index SYS.IDX_DIXIT at 28-JUN-22 PM -04:00

DBMS_STATS: Finished table SYS.BIGTAB. at 28-JUN-22 PM -04:00

Once you are done with the stats tracing, close it immediately!

SQL> exec dbms_stats.set_global_prefs('TRACE',0);

PL/SQL procedure successfully completed.

Hope It Helped!
Prashant Dixit

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High stats collection time for partitioned tables after upgrade to 19c

Posted by FatDBA on July 2, 2022

Recently, while working on a database upgrade from 12c to 19c (19.15) one of my friend encountered a strange issue on the newly upgraded 19c database where the stats gathering on the full database started taking huge time. It used to take ~ 3 hours to complete the full database statistics, but the same stats collection job after the upgrade to 19c started taking close to 10 hours. The stats collection script they were using was quite simple and with minimal parameters used.


During the analysis he observed that the top 3-4 in-flight transactions during stats collection were related to the Index Statistics and were found doing ‘Index Fast Full Scan’, and all of them are on few of the large partitioned tables in the database. He discussed the case with me and together tried few thing i.e. recollected dictionary and fixed object statistics, did some comparative study of parameters between 12c and 19c but none of them worked. At last we tried to set debugging levels on DBMS_STATS to see what’s happening under the hood, and that gave us some hint when set it with level/flag 8 (trace index stats) and with level 32768 to trace approximate NDV (number distinct values) gatherings. Traces gave us some idea that its surely with the index stats and NDV or number of distinct keys and is taking time.

But even after that we both were totally clueless as these Tables and its dependent objects are there in the system for a very long time. So. the big question was – What’s new in 19c that has slowed down stats collection ?

Finally we decided to contact OCS! And they quickly responded to the problem as its a known problem with the 19c. As per them, there was an enhancement in 19c that is related to Index stats gathering, and that had lead to the longer stats times. It was all due to an unpublished Bug 33427856 which is an enhancement to improve the calculation of index NDK (Number of Distinct Keys). This new feature with the approx_count_distinct function and fully scans indexes to calculate NDK. This has a significant benefit because NDK is now accurate. It also means that gathering statistics can take longer (for example, updating global index statistics if incremental stats is used). So, In general, this is expected behavior, since DBMS_STATS is doing more work in 19c than it did in previously unenhanced versions.

And the solution to this new 19c index-stats feature (a problem) off by setting fix control to disable ‘Enhance Index NDK Statistics’ – 27268249

alter system set "_fix_control"='27268249:0';

And as soon as we deleted existing statistics and regather them, the time dropped drastically and got completed under 3 hours.

Hope It Helped!
Prashant Dixit

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