CASE Condition Causing Plan Difference

Question

Version
SQL> SELECT * FROM v$version;
BANNER
Oracle Database 11g Enterprise Edition Release 11.2.0.2.0 - 64bit Production
PL/SQL Release 11.2.0.2.0 - Production
CORE    11.2.0.2.0      Production
TNS for Linux: Version 11.2.0.2.0 - Production
NLSRTL Version 11.2.0.2.0 - Production
5 rows selected.
Optimizer
SQL> show parameter optimizer
NAME                                 TYPE        VALUE
_optimizer_autostats_job             boolean     FALSE
_optimizer_join_elimination_enabled  boolean     FALSE
optimizer_capture_sql_plan_baselines boolean     FALSE
optimizer_dynamic_sampling           integer     2
optimizer_features_enable            string      11.2.0.2
optimizer_index_caching              integer     0
optimizer_index_cost_adj             integer     100
optimizer_mode                       string      ALL_ROWS
optimizer_secure_view_merging        boolean     FALSE
optimizer_use_invisible_indexes      boolean     FALSE
optimizer_use_pending_statistics     boolean     FALSE
optimizer_use_sql_plan_baselines     boolean     TRUE
Problem
We have a vendor system that constructs the SQL to be sent to the database. These queries have the following generic structure for a WHERE clause condition:
CASE WHEN <condition> THEN <table>.<column> END = '<value>'We've noticed significantly different execution plans when the optimizer compares numbers instead of varchar2. The examples below demonstrate that. In our case this difference is impacting the execution plans of a larger query effectively doubling buffer gets and resulting in execution times that are 2-4 times as worse.
Query 1
SQL> SELECT /*+gather_plan_statistics*/ * FROM DUAL WHERE CASE WHEN 1=1 THEN 'X' END = dummy;
D
X
1 row selected.
SQL> SELECT * FROM TABLE(DBMS_XPLAN.DISPLAY_CURSOR(null,null,'ALLSTATS LAST'));
PLAN_TABLE_OUTPUT
SQL_ID  6ghjubgpwpr61, child number 0
SELECT /*+gather_plan_statistics*/ * FROM DUAL WHERE CASE WHEN 1=1 THEN
'X' END = dummy
Plan hash value: 272002086
| Id  | Operation         | Name | Starts | E-Rows | A-Rows |   A-Time   | Buffers | Reads  |
|   0 | SELECT STATEMENT  |      |      1 |        |      1 |00:00:00.02 |       2 |      2 |
|*  1 |  TABLE ACCESS FULL| DUAL |      1 |      1 |      1 |00:00:00.02 |       2 |      2 |
Predicate Information (identified by operation id):
   1 - filter("DUMMY"='X')
Query 2
SQL> SELECT /*+gather_plan_statistics*/ * FROM DUAL WHERE CASE WHEN 'A'='A' THEN 'X' END = dummy;
D
X
1 row selected.
SQL> SELECT * FROM TABLE(DBMS_XPLAN.DISPLAY_CURSOR(null,null,'ALLSTATS LAST'));
PLAN_TABLE_OUTPUT
SQL_ID  gcpwzksqr2w9n, child number 0
SELECT /*+gather_plan_statistics*/ * FROM DUAL WHERE CASE WHEN 'A'='A'
THEN 'X' END = dummy
Plan hash value: 272002086
| Id  | Operation         | Name | Starts | E-Rows | A-Rows |   A-Time   | Buffers |
|   0 | SELECT STATEMENT  |      |      1 |        |      1 |00:00:00.01 |       2 |
|*  1 |  TABLE ACCESS FULL| DUAL |      1 |      1 |      1 |00:00:00.01 |       2 |
Predicate Information (identified by operation id):
   1 - filter("DUMMY"=CASE  WHEN ('A'='A') THEN 'X' END )In my eyes the condition 'A' = 'A' is equivalent to the condition 1=1. How come in the case of the numeric comparison Oracle can eliminate the case expression but in the character comparison it cannot?
Thanks!

Answer 1

Centinul wrote:
In my eyes the condition 'A' = 'A' is equivalent to the condition 1=1. Well, issue is Oracle looks at 'A' = 'A' as string comparison. And string comparison result is NLS setting dependent. That's why it can't evaluate CASE at compile time.
SY.