Hello Listers, We have encountered slowness with one of the customer
databases with a simple SELECT query. And analyzing the plan and outline of
the sql it appeared that while we were on version 11.2.0.4, it was using
index skip scan and was faster also the cost was lower but post upgrade to
19.9.0.0.0, there are few underscore parameters seems to be disabled along
with one fix control and thus it followed a full scan path. So we want to
understand if it's expected/ defaults in 19C? or it must have been altered
somehow during upgrade itself and so should be reverted back to true?
Below underscore parameters appear to be turned off as shown in the
query outline.
OPT_PARAM('_unnest_subquery' 'false')
OPT_PARAM('_optimizer_skip_scan_enabled' 'false')
OPT_PARAM('_gby_hash_aggregation_enabled' 'false')
********* Plan on 19.9.0.0.0 *********************
SQL Text
------------------------------
SELECT D.COL1, D.CLOB2 FROM TAB1 D WHERE D.ID = :1
Global Information
------------------------------
Status : DONE (ALL ROWS)
Instance ID : 1
SQL Execution ID : 16782261
Execution Started : 10/19/2021 12:38:18
First Refresh Time : 10/19/2021 12:38:22
Last Refresh Time : 10/19/2021 12:39:25
Duration : 67s
Fetch Calls : 1
Global Stats
===============================================================================
| Elapsed | Cpu | IO | Application | Fetch | Buffer | Read |
Read |
| Time(s) | Time(s) | Waits(s) | Waits(s) | Calls | Gets | Reqs |
Bytes |
===============================================================================
| 72 | 18 | 53 | 0.00 | 1 | 11M | 87343 |
85GB |
===============================================================================
SQL Plan Monitoring Details (Plan Hash Value=1148202243)
==========================================================================================================================================================================
| Id | Operation | Name | Rows |
Cost | Time | Start | Execs | Rows | Read | Read | Activity |
Activity Detail |
| | | | (Estim) |
| Active(s) | Active | | (Actual) | Reqs | Bytes | (%) |
(# samples) |
==========================================================================================================================================================================
| 0 | SELECT STATEMENT | | |
| 64 | +4 | 1 | 1 | | | |
|
| 1 | TABLE ACCESS STORAGE FULL | TAB1 | 1 |
2M | 68 | +1 | 1 | 1 | 87343 | 85GB | 100.00 | Cpu
(22) |
| | | | |
| | | | | | | |
direct path read (46) |
==========================================================================================================================================================================
Outline Data
-------------
/*+
BEGIN_OUTLINE_DATA
IGNORE_OPTIM_EMBEDDED_HINTS
OPTIMIZER_FEATURES_ENABLE('19.1.0')
DB_VERSION('19.1.0')
OPT_PARAM('_unnest_subquery' 'false')
OPT_PARAM('_optimizer_skip_scan_enabled' 'false')
OPT_PARAM('_gby_hash_aggregation_enabled' 'false')
OPT_PARAM('_fix_control' '14033181:0')
ALL_ROWS
OUTLINE_LEAF(@"SEL$1")
FULL(@"SEL$1" "D"@"SEL$1")
END_OUTLINE_DATA
*/
********* Plan on 11.2.0.4 *********************
SQL Monitoring Report
SQL Text
------------------------------
SELECT /*+optimizer_features_enable('11.2.0.4') */ D.COL1, D.CLOB2 FROM
TAB1 D WHERE D.ID = :b1
Global Information
------------------------------
Status : DONE (ALL ROWS)
Instance ID : 1
SQL ID : 09sa822k7qsvn
SQL Execution ID : 16777216
Execution Started : 10/19/2021 15:13:56
First Refresh Time : 10/19/2021 15:13:56
Last Refresh Time : 10/19/2021 15:13:56
Duration : .032143s
Module/Action : SQL*Plus/-
Fetch Calls : 2
Global Stats
===========================================================================
| Elapsed | Cpu | IO | Cluster | Fetch | Buffer | Read | Read |
| Time(s) | Time(s) | Waits(s) | Waits(s) | Calls | Gets | Reqs | Bytes |
===========================================================================
| 0.03 | 0.01 | 0.02 | 0.00 | 2 | 54 | 13 | 104KB |
===========================================================================
SQL Plan Monitoring Details (Plan Hash Value=191420461)
========================================================================================================================================================================
| Id | Operation | Name | Rows
| Cost | Time | Start | Execs | Rows | Read | Read | Activity |
Activity Detail |
| | | | (Estim)
| | Active(s) | Active | | (Actual) | Reqs | Bytes | (%) |
(# samples) |
========================================================================================================================================================================
| 0 | SELECT STATEMENT | |
| | 1 | +0 | 1 | 1 | | | |
|
| 1 | TABLE ACCESS BY INDEX ROWID | TAB1 | 1
| 6 | 1 | +0 | 1 | 1 | 1 | 8192 | |
|
| 2 | INDEX SKIP SCAN | TAB1_PK | 1
| 5 | 1 | +0 | 1 | 1 | 12 | 98304 | |
|
========================================================================================================================================================================
Query Block Name / Object Alias (identified by operation id):
-------------------------------------------------------------
1 - SEL$1 / D@SEL$1
2 - SEL$1 / D@SEL$1
Outline Data
-------------
/*+
BEGIN_OUTLINE_DATA
IGNORE_OPTIM_EMBEDDED_HINTS
OPTIMIZER_FEATURES_ENABLE('11.2.0.4')
DB_VERSION('19.1.0')
ALL_ROWS
OUTLINE_LEAF(@"SEL$1")
INDEX_SS(@"SEL$1" "D"@"SEL$1" ("TAB1"."COL3" "TAB1"."ID"))
END_OUTLINE_DATA
*/
Predicate Information (identified by operation id):
---------------------------------------------------
2 - access("D"."ID"=:B1)
filter("D"."ID"=:B1)
Column Projection Information (identified by operation id):
-----------------------------------------------------------
1 - "D"."COL1"[VARCHAR2,19], "D"."CLOB2"[LOB,4000]
2 - "D".ROWID[ROWID,10]
