A customer complained that a query and its INSERT counterpart had different execution plans, and of course, the INSERT was slower. First lets look at the SELECT : SELECT ua_tr_rundatetime,        ua_ch_treatmentcode,        ua_tr_treatmentcode,        ua_ch_cellid,        ua_tr_cellid FROM   (SELECT DISTINCT CH.treatmentcode AS UA_CH_TREATMENTCODE,                         CH.cellid        AS UA_CH_CELLID         FROM    CH,                 DL         WHERE  CH.contactdatetime > SYSDATE - 5                AND CH.treatmentcode = DL.treatmentcode) CH_CELLS,        (SELECT DISTINCT T.treatmentcode AS UA_TR_TREATMENTCODE,                         T.cellid        AS UA_TR_CELLID,                         T.rundatetime   AS UA_TR_RUNDATETIME         FROM    T,                 DL         WHERE  T.treatmentcode = DL.treatmentcode) TRT_CELLS WHERE  CH_CELLS.ua_ch_treatmentcode(+) = TRT_CELLS.ua_tr_treatmentcode;  The query has 2 DISTINCT subqueries.  The execution plan shows one with DISTICT Placement transformation applied and not the other. The view in Step 5 has the prefix VW_DTP which means DISTINCT Placement. -------------------------------------------------------------------- | Id  | Operation                    | Name            | Cost (%CPU) -------------------------------------------------------------------- |   0 | SELECT STATEMENT             |                 |   272K(100) |*  1 |  HASH JOIN OUTER             |                 |   272K  (1) |   2 |   VIEW                       |                 |  4408   (1) |   3 |    HASH UNIQUE               |                 |  4408   (1) |*  4 |     HASH JOIN                |                 |  4407   (1) |   5 |      VIEW                    | VW_DTP_48BAF62C |  1660   (2) |   6 |       HASH UNIQUE            |                 |  1660   (2) |   7 |        TABLE ACCESS FULL     | DL              |  1644   (1) |   8 |      TABLE ACCESS FULL       | T               |  2744   (1) |   9 |   VIEW                       |                 |   267K  (1) |  10 |    HASH UNIQUE               |                 |   267K  (1) |* 11 |     HASH JOIN                |                 |   267K  (1) |  12 |      PARTITION RANGE ITERATOR|                 |   266K  (1) |* 13 |       TABLE ACCESS FULL      | CH              |   266K  (1) |  14 |      TABLE ACCESS FULL       | DL              |  1644   (1) -------------------------------------------------------------------- Query Block Name / Object Alias (identified by operation id): -------------------------------------------------------------    1 - SEL$1    2 - SEL$AF418D5F / TRT_CELLS@SEL$1    3 - SEL$AF418D5F    5 - SEL$F6AECEDE / VW_DTP_48BAF62C@SEL$48BAF62C    6 - SEL$F6AECEDE    7 - SEL$F6AECEDE / DL@SEL$3    8 - SEL$AF418D5F / T@SEL$3    9 - SEL$2        / CH_CELLS@SEL$1   10 - SEL$2   13 - SEL$2        / CH@SEL$2   14 - SEL$2        / DL@SEL$2 Predicate Information (identified by operation id): ---------------------------------------------------    1 - access("CH_CELLS"."UA_CH_TREATMENTCODE"="TRT_CELLS"."UA_TR_TREATMENTCODE")    4 - access("T"."TREATMENTCODE"="ITEM_1")   11 - access("CH"."TREATMENTCODE"="DL"."TREATMENTCODE")   13 - filter("CH"."CONTACTDATETIME">SYSDATE@!-5) The outline shows PLACE_DISTINCT(@"SEL$3" "DL"@"SEL$3") indicating that the QB3 is the one that got the transformation. Outline Data -------------   /*+       BEGIN_OUTLINE_DATA       IGNORE_OPTIM_EMBEDDED_HINTS       OPTIMIZER_FEATURES_ENABLE('11.2.0.3')       DB_VERSION('11.2.0.3')       ALL_ROWS       OUTLINE_LEAF(@"SEL$2")       OUTLINE_LEAF(@"SEL$F6AECEDE")       OUTLINE_LEAF(@"SEL$AF418D5F") PLACE_DISTINCT(@"SEL$3" "DL"@"SEL$3")       OUTLINE_LEAF(@"SEL$1")       OUTLINE(@"SEL$48BAF62C")       OUTLINE(@"SEL$3")       NO_ACCESS(@"SEL$1" "TRT_CELLS"@"SEL$1")       NO_ACCESS(@"SEL$1" "CH_CELLS"@"SEL$1")       LEADING(@"SEL$1" "TRT_CELLS"@"SEL$1" "CH_CELLS"@"SEL$1")       USE_HASH(@"SEL$1" "CH_CELLS"@"SEL$1")       FULL(@"SEL$2" "CH"@"SEL$2")       FULL(@"SEL$2" "DL"@"SEL$2")       LEADING(@"SEL$2" "CH"@"SEL$2" "DL"@"SEL$2")       USE_HASH(@"SEL$2" "DL"@"SEL$2")       USE_HASH_AGGREGATION(@"SEL$2")       NO_ACCESS(@"SEL$AF418D5F" "VW_DTP_48BAF62C"@"SEL$48BAF62C")       FULL(@"SEL$AF418D5F" "T"@"SEL$3")       LEADING(@"SEL$AF418D5F" "VW_DTP_48BAF62C"@"SEL$48BAF62C" "T"@"SEL$3")       USE_HASH(@"SEL$AF418D5F" "T"@"SEL$3")       USE_HASH_AGGREGATION(@"SEL$AF418D5F")       FULL(@"SEL$F6AECEDE" "DL"@"SEL$3")       USE_HASH_AGGREGATION(@"SEL$F6AECEDE")       END_OUTLINE_DATA   */ The 10053 shows there is a comparative of cost with and without the transformation. This means the transformation belongs to Cost-Based Query Transformations (CBQT). In SEL$3 the optimization of the query block without the transformation is 6659.73 and with the transformation is 4408.41 so the transformation is kept. GBP/DP: Checking validity of GBP/DP for query block SEL$3 (#3) DP: Checking validity of distinct placement for query block SEL$3 (#3) DP: Using search type: linear DP: Considering distinct placement on query block SEL$3 (#3) DP: Starting iteration 1, state space = (5) : (0) DP: Original query DP: Costing query block. DP: Updated best state, Cost = 6659.73 DP: Starting iteration 2, state space = (5) : (1) DP: Using DP transformation in this iteration. DP: Transformed query DP: Costing query block. DP: Updated best state, Cost = 4408.41 DP: Doing DP on the original QB. DP: Doing DP on the preserved QB. In SEL$2 the cost without the transformation is less than with it so it is not kept. GBP/DP: Checking validity of GBP/DP for query block SEL$2 (#2) DP: Checking validity of distinct placement for query block SEL$2 (#2) DP: Using search type: linear DP: Considering distinct placement on query block SEL$2 (#2) DP: Starting iteration 1, state space = (3) : (0) DP: Original query DP: Costing query block. DP: Updated best state, Cost = 267936.93 DP: Starting iteration 2, state space = (3) : (1) DP: Using DP transformation in this iteration. DP: Transformed query DP: Costing query block. DP: Not update best state, Cost = 267951.66 To the same query an INSERT INTO is added and the result is a very different execution plan. INSERT  INTO cc               (ua_tr_rundatetime,                ua_ch_treatmentcode,                ua_tr_treatmentcode,                ua_ch_cellid,                ua_tr_cellid)SELECT ua_tr_rundatetime,       ua_ch_treatmentcode,       ua_tr_treatmentcode,       ua_ch_cellid,       ua_tr_cellidFROM   (SELECT DISTINCT CH.treatmentcode AS UA_CH_TREATMENTCODE,                        CH.cellid        AS UA_CH_CELLID        FROM    CH,                DL        WHERE  CH.contactdatetime > SYSDATE - 5               AND CH.treatmentcode = DL.treatmentcode) CH_CELLS,       (SELECT DISTINCT T.treatmentcode AS UA_TR_TREATMENTCODE,                        T.cellid        AS UA_TR_CELLID,                        T.rundatetime   AS UA_TR_RUNDATETIME        FROM    T,                DL        WHERE  T.treatmentcode = DL.treatmentcode) TRT_CELLSWHERE  CH_CELLS.ua_ch_treatmentcode(+) = TRT_CELLS.ua_tr_treatmentcode;----------------------------------------------------------| Id  | Operation                     | Name | Cost (%CPU)----------------------------------------------------------|   0 | INSERT STATEMENT              |      |   274K(100)|   1 |  LOAD TABLE CONVENTIONAL      |      |            |*  2 |   HASH JOIN OUTER             |      |   274K  (1)|   3 |    VIEW                       |      |  6660   (1)|   4 |     SORT UNIQUE               |      |  6660   (1)|*  5 |      HASH JOIN                |      |  6659   (1)|   6 |       TABLE ACCESS FULL       | DL   |  1644   (1)|   7 |       TABLE ACCESS FULL       | T    |  2744   (1)|   8 |    VIEW                       |      |   267K  (1)|   9 |     SORT UNIQUE               |      |   267K  (1)|* 10 |      HASH JOIN                |      |   267K  (1)|  11 |       PARTITION RANGE ITERATOR|      |   266K  (1)|* 12 |        TABLE ACCESS FULL      | CH   |   266K  (1)|  13 |       TABLE ACCESS FULL       | DL   |  1644   (1)----------------------------------------------------------Query Block Name / Object Alias (identified by operation id):-------------------------------------------------------------   1 - SEL$1   3 - SEL$3 / TRT_CELLS@SEL$1   4 - SEL$3   6 - SEL$3 / DL@SEL$3   7 - SEL$3 / T@SEL$3   8 - SEL$2 / CH_CELLS@SEL$1   9 - SEL$2  12 - SEL$2 / CH@SEL$2  13 - SEL$2 / DL@SEL$2Predicate Information (identified by operation id):---------------------------------------------------   2 - access("CH_CELLS"."UA_CH_TREATMENTCODE"="TRT_CELLS"."UA_TR_TREATMENTCODE")   5 - access("T"."TREATMENTCODE"="DL"."TREATMENTCODE")  10 - access("CH"."TREATMENTCODE"="DL"."TREATMENTCODE")  12 - filter("CH"."CONTACTDATETIME">SYSDATE@!-5)Outline Data-------------  /*+      BEGIN_OUTLINE_DATA      IGNORE_OPTIM_EMBEDDED_HINTS      OPTIMIZER_FEATURES_ENABLE('11.2.0.3')      DB_VERSION('11.2.0.3')      ALL_ROWS      OUTLINE_LEAF(@"SEL$2")      OUTLINE_LEAF(@"SEL$3")      OUTLINE_LEAF(@"SEL$1")      OUTLINE_LEAF(@"INS$1")      FULL(@"INS$1" "CC"@"INS$1")      NO_ACCESS(@"SEL$1" "TRT_CELLS"@"SEL$1")      NO_ACCESS(@"SEL$1" "CH_CELLS"@"SEL$1")      LEADING(@"SEL$1" "TRT_CELLS"@"SEL$1" "CH_CELLS"@"SEL$1")      USE_HASH(@"SEL$1" "CH_CELLS"@"SEL$1")      FULL(@"SEL$2" "CH"@"SEL$2")      FULL(@"SEL$2" "DL"@"SEL$2")      LEADING(@"SEL$2" "CH"@"SEL$2" "DL"@"SEL$2")      USE_HASH(@"SEL$2" "DL"@"SEL$2")      USE_HASH_AGGREGATION(@"SEL$2")      FULL(@"SEL$3" "DL"@"SEL$3")      FULL(@"SEL$3" "T"@"SEL$3")      LEADING(@"SEL$3" "DL"@"SEL$3" "T"@"SEL$3")      USE_HASH(@"SEL$3" "T"@"SEL$3")      USE_HASH_AGGREGATION(@"SEL$3")      END_OUTLINE_DATA  */ There is no DISTINCT Placement view and no hint.The 10053 trace shows a new legend "DP: Bypassed: Not SELECT"implying that this is a transformation that it is possible only for SELECTs. GBP/DP: Checking validity of GBP/DP for query block SEL$3 (#4) DP: Checking validity of distinct placement for query block SEL$3 (#4) DP: Bypassed: Not SELECT. GBP/DP: Checking validity of GBP/DP for query block SEL$2 (#3) DP: Checking validity of distinct placement for query block SEL$2 (#3) DP: Bypassed: Not SELECT. In 12.1 (and hopefully in 11.2.0.4 when released) the restriction on applying CBQT to some DMLs and DDLs (like CTAS) is lifted.This is documented in BugTag Note:10013899.8 Allow CBQT for some DML / DDLAnd interestingly enough, it is possible to have a one-off patch in 11.2.0.3. SQL> select DESCRIPTION,OPTIMIZER_FEATURE_ENABLE,IS_DEFAULT     2  from v$system_fix_control where BUGNO='10013899'; DESCRIPTION ---------------------------------------------------------------- OPTIMIZER_FEATURE_ENABLE  IS_DEFAULT ------------------------- ---------- enable some transformations for DDL and DML statements 11.2.0.4                           1