Tales From A Lazy Fat DBA

Hi Mates,
Today would like to discuss about the ‘SQL Plan Stability’, specially situations when the performance of a SQL statement degrades and performs poor in one environment and working absolutely good in other spheres.
I experienced such issues a lot during past few years while working as a performance consultant for many of the projects, but recently i encountered similar issue and would like to discuss about the same.
In short the situation is given below for that problematic SQL

Same request they say takes in SIT 268 sec:
• I run in SIT again –> 500 secs
• I run also in DEV –> 20 secs
• I run in preSIT –> 5 secs

Below are the details for the given Ill-SQL.

SQL Text:
SELECT a.port_inst_id AS mdf_es_port_inst_id, a.status AS mdf_es_port_status, d.port_inst_id AS mdf_ls_port_inst_id, d.status AS mdf_ls_port_status, regexp_substr( c.descr, :"SYS_B_00", :"SYS_B_01", :"SYS_B_02" ) AS naming_area, a.status AS mdf_es_port_status2, DECODE( a.reservation_inst_id, NULL, :"SYS_B_03", :"SYS_B_04" ) AS mdf_es_active_reservation, a.port_access_id AS lic_identity, egi_get_uda_value( a.equip_inst_id, :"SYS_B_05", :"SYS_B_06", :"SYS_B_07" ) AS kc, b.name AS coordinate, a.port_hum_id egi_port_name, egi_get_uda_value( d.equip_inst_id, :"SYS_B_08", :"SYS_B_09", :"SYS_B_10" ) AS cabinet_name, e.name AS termination_block_name, d.port_access_ id AS pair_name, egi_get_uda_value( d.equip_inst_id, :"SYS_B_11", :"SYS_B_12", :"SYS_B_13" ) AS signing_system, egi_get_uda_value( a.port_inst_id, :"SYS_B_14", :"SYS_B_15", :"SYS_B_16" ) AS processor_exchange, a.circ_path_inst_id AS es_circ_path_inst_id, a.next_path_inst_id AS es_next_path_inst_id, d.circ_path_inst_id AS ls_circ_path_inst_id, d.next_path_inst_id AS ls_next_path_inst_id, DECODE( d.role, :"SYS_B_17", :"SYS_B_18", :"SYS_B_19", :"SYS_B_20", :"SYS_B_21", :"SYS_B_22", :"SYS_B_23", :"SYS_B_24", NULL ) AS cabling_type FROM epa a, card_inst b, equip_inst c, epa d, card_inst e, xxx_xxxx_xxxxxxxx f WHERE a.port_inst_id = f.port_inst_id AND f.val_attr_inst_id = ( SELECT val_attr_inst_id FROM xxx_xxxx_xxxxxxxx WHERE group_name = :"SYS_B_25" AND attr_name = :"SYS_B_26" ) AND a.card_inst_id = b.card_inst_id AND b.type = :"SYS_B_27" AND a.equip_inst_id = c.equip_inst_id AND a.z_wired_port_inst_id = d.port_inst_id (+) AND d.card_inst_id = e.card_inst_id (+) AND regexp_substr( a.port_access_id, :"SYS_B_28", :"SYS_B_29", :"SYS_B_30" ) = :"SYS_B_31" AND to_number(regexp_substr( a.port_access_id, :"SYS_B_32", :"SYS_B_33", :"SYS_B_34" ) ) >= to_number(: "SYS_B_35") AND to_number(regexp_substr( a.port_access_id, :"SYS_B_36", :"SYS_B_37", :"SYS_B_38" ) ) <= to_number(:"SYS_B_39") AND f.attr_value = :"SYS_B_40" ORDER BY :"SYS_B_41"

I see around 5 different PHV’s or plans coming for this statement and last two 18733286 and 2202641467 are the worst plans here, first one with PHV 1663132818 is the BEST among all others.
1663132818 is with very less elapsed time, CPU Usage, IO time, LIOs, Disk reads, Rows Processed and Cost.
** From DBA_HIST_SQLSTAT view.

Let’s collect the OUTLINE and QUERY Block details of the statement, and this can be collected using
select * from table(dbms_xplan.display_awr(‘SQL_ID’, PHV, format => ‘ADVANCED’));

Query Block Name / Object Alias (identified by operation id):
————————————————————-

1 - SEL$841DDE77
10 - SEL$841DDE77 / VAL_ATTR_NAME@SEL$2
11 - SEL$841DDE77 / VAL_ATTR_NAME@SEL$2
12 - SEL$841DDE77 / F@SEL$1
13 - SEL$841DDE77 / F@SEL$1
14 - SEL$841DDE77 / A@SEL$1
15 - SEL$841DDE77 / A@SEL$1
16 - SEL$841DDE77 / C@SEL$1
17 - SEL$841DDE77 / C@SEL$1
18 - SEL$841DDE77 / D@SEL$1
19 - SEL$841DDE77 / D@SEL$1
20 - SEL$841DDE77 / E@SEL$1
21 - SEL$841DDE77 / E@SEL$1
22 - SEL$841DDE77 / B@SEL$1
23 - SEL$841DDE77 / B@SEL$1

Outline Data
————-
/*+
BEGIN_OUTLINE_DATA
IGNORE_OPTIM_EMBEDDED_HINTS
OPTIMIZER_FEATURES_ENABLE('12.1.0.2')
DB_VERSION('12.1.0.2')
OPT_PARAM('optimizer_index_cost_adj' 10)
FIRST_ROWS(1)
OUTLINE_LEAF(@"SEL$841DDE77")
MERGE(@"SEL$683B0107")
OUTLINE(@"SEL$C772B8D1")
UNNEST(@"SEL$2")
OUTLINE(@"SEL$683B0107")
OUTLINE(@"SEL$7511BFD2")
OUTLINE(@"SEL$2")
OUTLINE(@"SEL$1")
INDEX_RS_ASC(@"SEL$841DDE77" "VAL_ATTR_NAME"@"SEL$2" ("VAL_ATTR_NAME"."GROUP_NAME" "VAL_ATTR_NAME"."ATTR_NAME"))
INDEX_RS_ASC(@"SEL$841DDE77" "F"@"SEL$1" ("PORT_ATTR_SETTINGS"."ATTR_VALUE"))
BATCH_TABLE_ACCESS_BY_ROWID(@"SEL$841DDE77" "F"@"SEL$1")
INDEX_RS_ASC(@"SEL$841DDE77" "A"@"SEL$1" ("EPA"."PORT_INST_ID"))
INDEX_RS_ASC(@"SEL$841DDE77" "C"@"SEL$1" ("EQUIP_INST"."EQUIP_INST_ID"))
INDEX_RS_ASC(@"SEL$841DDE77" "D"@"SEL$1" ("EPA"."PORT_INST_ID"))
INDEX_RS_ASC(@"SEL$841DDE77" "E"@"SEL$1" ("CARD_INST"."CARD_INST_ID"))
INDEX(@"SEL$841DDE77" "B"@"SEL$1" ("CARD_INST"."CARD_INST_ID"))
LEADING(@"SEL$841DDE77" "VAL_ATTR_NAME"@"SEL$2" "F"@"SEL$1" "A"@"SEL$1" "C"@"SEL$1" "D"@"SEL$1" "E"@"SEL$1"
"B"@"SEL$1")
USE_NL(@"SEL$841DDE77" "F"@"SEL$1")
USE_NL(@"SEL$841DDE77" "A"@"SEL$1")
USE_NL(@"SEL$841DDE77" "C"@"SEL$1")
USE_NL(@"SEL$841DDE77" "D"@"SEL$1")
USE_NL(@"SEL$841DDE77" "E"@"SEL$1")
USE_NL(@"SEL$841DDE77" "B"@"SEL$1")
NLJ_BATCHING(@"SEL$841DDE77" "B"@"SEL$1")
END_OUTLINE_DATA
*/

Let’s use the outline of the good plan as a hint to force the good execution plan to be used and check the execution time for the query.
Run the query from SQLPLUS using the hint:

SELECT
/*+
BEGIN_OUTLINE_DATA
IGNORE_OPTIM_EMBEDDED_HINTS
OPTIMIZER_FEATURES_ENABLE(‘12.1.0.2’)
DB_VERSION(‘12.1.0.2’)
OPT_PARAM(‘optimizer_index_cost_adj’ 10)
FIRST_ROWS(1)
OUTLINE_LEAF(@”SEL$841DDE77″)
MERGE(@”SEL$683B0107″)
OUTLINE(@”SEL$C772B8D1″)
UNNEST(@”SEL$2″)
OUTLINE(@”SEL$683B0107″)
OUTLINE(@”SEL$7511BFD2″)
OUTLINE(@”SEL$2″)
OUTLINE(@”SEL$1″)
INDEX_RS_ASC(@”SEL$841DDE77″ “VAL_ATTR_NAME”@”SEL$2” (“VAL_ATTR_NAME”.”GROUP_NAME” “VAL_ATTR_NAME”.”ATTR_NAME”))
INDEX_RS_ASC(@”SEL$841DDE77″ “F”@”SEL$1” (“PORT_ATTR_SETTINGS”.”ATTR_VALUE”))
BATCH_TABLE_ACCESS_BY_ROWID(@”SEL$841DDE77″ “F”@”SEL$1″)
INDEX_RS_ASC(@”SEL$841DDE77” “A”@”SEL$1” (“EPA”.”PORT_INST_ID”))
INDEX_RS_ASC(@”SEL$841DDE77″ “C”@”SEL$1” (“EQUIP_INST”.”EQUIP_INST_ID”))
INDEX_RS_ASC(@”SEL$841DDE77″ “D”@”SEL$1” (“EPA”.”PORT_INST_ID”))
INDEX_RS_ASC(@”SEL$841DDE77″ “E”@”SEL$1” (“CARD_INST”.”CARD_INST_ID”))
INDEX(@”SEL$841DDE77″ “B”@”SEL$1” (“CARD_INST”.”CARD_INST_ID”))
LEADING(@”SEL$841DDE77″ “VAL_ATTR_NAME”@”SEL$2” “F”@”SEL$1” “A”@”SEL$1” “C”@”SEL$1” “D”@”SEL$1” “E”@”SEL$1”
“B”@”SEL$1″)
USE_NL(@”SEL$841DDE77” “F”@”SEL$1″)
USE_NL(@”SEL$841DDE77” “A”@”SEL$1″)
USE_NL(@”SEL$841DDE77” “C”@”SEL$1″)
USE_NL(@”SEL$841DDE77” “D”@”SEL$1″)
USE_NL(@”SEL$841DDE77” “E”@”SEL$1″)
USE_NL(@”SEL$841DDE77” “B”@”SEL$1″)
NLJ_BATCHING(@”SEL$841DDE77” “B”@”SEL$1”)
END_OUTLINE_DATA
*/
A.PORT_INST_ID AS MDF_ES_PORT_INST_ID, A.STATUS AS MDF_ES_PORT_STATUS, D.PORT_INST_ID AS MDF_LS_PORT_INST_ID, D.STATUS AS MDF_LS_PORT_STATUS, REGEXP_SUBSTR(C.DESCR,'[^ ]+’, 1,1) AS NAMING_AREA, A.STATUS AS MDF_ES_PORT_STATUS2, DECODE(A.RESERVATION_INST_ID, NULL, ‘N’, ‘Y’) AS MDF_ES_ACTIVE_RESERVATION, A.PORT_ACCESS_ID AS LIC_IDENTITY, EGI_GET_UDA_VALUE(A.EQUIP_INST_ID,’EQUIPMENT’,’Eqp Legacy Names’,’TAD-N’) AS KC, B.NAME AS COORDINATE, A.PORT_HUM_ID EGI_PORT_NAME, EGI_GET_UDA_VALUE(D.EQUIP_INST_ID,’EQUIPMENT’,’Eqp Legacy Names’,’TAD-N’) AS CABINET_NAME, E.NAME AS TERMINATION_BLOCK_NAME, D.PORT_ACCESS_ID AS PAIR_NAME, EGI_GET_UDA_VALUE(D.EQUIP_INST_ID,’EQUIPMENT’,’Eqp Legacy Names’,’SIGNSYST’) AS SIGNING_SYSTEM, EGI_GET_UDA_VALUE(A.PORT_INST_ID,’PORT’,’Copper Port Info’,’Processor Exchange’) AS PROCESSOR_EXCHANGE, A.CIRC_PATH_INST_ID AS ES_CIRC_PATH_INST_ID, A.NEXT_PATH_INST_ID AS ES_NEXT_PATH_INST_ID, D.CIRC_PATH_INST_ID AS LS_CIRC_PATH_INST_ID, D.NEXT_PATH_INST_ID AS LS_NEXT_PATH_INST_ID, DECODE(D.ROLE,’Primary’,’P’,’Secondary’,’S’,’Spare’,’R’,’Junction’,’F’,NULL) AS CABLING_TYPE FROM EPA A, CARD_INST B, EQUIP_INST C, EPA D, CARD_INST E, PORT_ATTR_SETTINGS F WHERE A.PORT_INST_ID =F.PORT_INST_ID AND F.VAL_ATTR_INST_ID= (SELECT VAL_ATTR_INST_ID FROM VAL_ATTR_NAME WHERE GROUP_NAME=’Copper Port Info’ AND ATTR_NAME =’Processor Exchange’ ) AND A.CARD_INST_ID =B.CARD_INST_ID AND B.TYPE =’ES’ AND A.EQUIP_INST_ID =C.EQUIP_INST_ID AND A.Z_WIRED_PORT_INST_ID =D.PORT_INST_ID(+) AND D.CARD_INST_ID =E.CARD_INST_ID(+) AND REGEXP_SUBSTR(A.PORT_ACCESS_ID,'[^-]+’, 1,1) =:B4 AND TO_NUMBER(REGEXP_SUBSTR(A.PORT_ACCESS_ID,'[^-]+’, 1,2))>=TO_NUMBER(:B3 ) AND TO_NUMBER(REGEXP_SUBSTR(A.PORT_ACCESS_ID,'[^-]+’, 1,2))<=TO_NUMBER(:B2 ) AND F.ATTR_VALUE =:B1 ORDER BY 8

I’ve executed the query in SIT and it’s taking no more than 5 seconds now with those OUTLINES, that is reeeeally good 😊

Now when the best plan is identified and tested, Question comes “HOW TO FIX THIS PERMANENTLY ??”

This can be done using one of the Oracle provided script named “coe_xfr_sql_profile.sql” which is part of popular SQLTXPLAIN/SQLT tool.
Download it from Metalink website (Direct Link: https://support.oracle.com/epmos/faces/DocumentDisplay?_afrLoop=458593899434965&parent=DOCUMENT&sourceId=1955195.1&id=215187.1&_afrWindowMode=0&_adf.ctrl-state=11co9htqxp_387#aref_section13)

In order to fix or correct the cost estimates for CBO to adopt a good execution plan we can follow below steps.
• Run the script coe_xfr_sql_profile.sql as SYSDBA user providing the sql_id and the good Plan Hash Value (PHV)
SQL> START coe_xfr_sql_profile.sql SQL_ID PHV

• Above step generates a script named in the format: “coe_xfr_sql_profile_SQL_ID_PLAN_HASH_VALUE.sql” i.e. including the sql_id and plan hash value.
Example: “coe_xfr_sql_profile_ SQLID_PHV.sql”

• If you like to use the profile for all literals, you can modify the force_match parameter in the generated script from false to true: force_match => TRUE
This will ensure the profile will be enabled even when different literals are used in the SQL query.

• Run the generated script as SYSDBA user in order to correct the optimizer cost estimates used in the execution plan for the sql_id and encourage it to use the desired plan obtained from the stated plan hash value.

• Next we can check if profile is accepted successfully
select SQL_ID, SQL_PROFILE,PLAN_HASH_VALUE from V$SQL where SQL_ID=’ SQL ID’;

• In case if profile is required to be disabled/dropped then do following steps.
The profile can be disabled or dropped as follows using the the SQL_PROFILE name returned from the query above:

EXEC DBMS_SQLTUNE.ALTER_SQL_PROFILE(Name => ”, Attribute_Name => ‘STATUS’, Value => ‘DISABLED’);

To Drop the Profile:
EXEC DBMS_SQLTUNE.DROP_SQL_PROFILE(Name => ”);

This way we can test and fix a Plan Stability issues with any SQL statement!

Hope It Helps!
Prashant Dixit