Enabling Parallel DML
A DML statement can be parallelized only if you have explicitly enabled parallel DML in the session, as in the following statement:
关于dml并行必须在会话层面明确指定,不指定都是位并行
ALTER SESSION ENABLE PARALLEL DML;
下面查看例子
create table scott.t1 as select b.OBJECT_ID,b.OBJECT_NAME from dba_objects b ; SQL> select count(1) from scott.t1; COUNT(1) ---------- 608657 SQL> update /*+ parallel(a,2) */ scott.t1 a set OBJECT_NAME='hthorizon.com'; 608657 rows updated. Elapsed: 00:00:37.12 alter system flush buffer_cache; alter system flush shared_pool; SQL> alter session enable parallel dml; Session altered. Elapsed: 00:00:00.03 SQL> update /*+ parallel(a,2) */ scott.t1 a set OBJECT_NAME='hthorizon.com1'; 608657 rows updated. Elapsed: 00:00:13.56 真正的 parallel update更新比伪parallel块0.24秒,下面查看执行计划 SQL> explain plan for update /*+ parallel(a,2) */ scott.t1 a set OBJECT_NAME='hthorizon.com'; Explained. SQL> select * from table(dbms_xplan.display); PLAN_TABLE_OUTPUT ----------------------------------------------------------------------------------------------------------------------- Plan hash value: 121765358 --------------------------------------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | TQ |IN-OUT| PQ Distrib | --------------------------------------------------------------------------------------------------------------- | 0 | UPDATE STATEMENT | | 1391K| 33M| 1011 (1)| 00:00:13 | | | | | 1 | UPDATE | T1 | | | | | | | | | 2 | PX COORDINATOR | | | | | | | | | | 3 | PX SEND QC (RANDOM)| :TQ10000 | 1391K| 33M| 1011 (1)| 00:00:13 | Q1,00 | P->S | QC (RAND) | | 4 | PX BLOCK ITERATOR | | 1391K| 33M| 1011 (1)| 00:00:13 | Q1,00 | PCWC | | | 5 | TABLE ACCESS FULL| T1 | 1391K| 33M| 1011 (1)| 00:00:13 | Q1,00 | PCWP | | PLAN_TABLE_OUTPUT -------------------------------------------------------------------------------------------------------------------- 上面并行更新时伪并行,只有在select时并行,真正的更新并行是发生在P->S(parallel_to_serial)之后;更新慢是慢在QC(query coordinator/查询协调)之上 SQL> alter session enable parallel dml; Session altered. SQL> explain plan for update /*+ parallel(a,2) */ scott.t1 a set OBJECT_NAME='hthorizon.com'; Explained. SQL> select * from table(dbms_xplan.display); PLAN_TABLE_OUTPUT ------------------------------------------------------------------------------------------------------------------------------------------------------ Plan hash value: 3991856572 --------------------------------------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | TQ |IN-OUT| PQ Distrib | --------------------------------------------------------------------------------------------------------------- | 0 | UPDATE STATEMENT | | 1391K| 33M| 1011 (1)| 00:00:13 | | | | | 1 | PX COORDINATOR | | | | | | | | | | 2 | PX SEND QC (RANDOM) | :TQ10000 | 1391K| 33M| 1011 (1)| 00:00:13 | Q1,00 | P->S | QC (RAND) | | 3 | UPDATE | T1 | | | | | Q1,00 | PCWP | | | 4 | PX BLOCK ITERATOR | | 1391K| 33M| 1011 (1)| 00:00:13 | Q1,00 | PCWC | | | 5 | TABLE ACCESS FULL| T1 | 1391K| 33M| 1011 (1)| 00:00:13 | Q1,00 | PCWP | | PLAN_TABLE_OUTPUT ------------------------------------------------------------------------------------------------------------------------------------------------------ 从上述执行计划来看是真正的update并行 查看sql的实际执行计划(执行过的语句才能生成真实的执行计划) SQL> select a.SQL_ID,a.EXECUTIONS,a.CHILD_number from v$sql a where a.SQL_TEXT like 'update /*+ parallel(a,2) */ scott.t1%'; SQL_ID EXECUTIONS CHILD_NUMBER ------------- ---------- ------------ 9fwr6kw5xwmq0 1 0 Elapsed: 00:00:00.00 SQL> select * from table(xplan.DISPLAY_cursor('9fwr6kw5xwmq0',0,'advanced')); PLAN_TABLE_OUTPUT ------------------------------------------------------------------------------------------------------------------------------------------------------ SQL_ID 9fwr6kw5xwmq0, child number 0 ------------------------------------- update /*+ parallel(a,2) */ scott.t1 a set OBJECT_NAME='hthorizon.com1' Plan hash value: 3991856572 ----------------------------------------------------------------------------------------------------------------------- | Id | Order | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | TQ |IN-OUT| PQ Distrib | ----------------------------------------------------------------------------------------------------------------------- | 0 | 6 | UPDATE STATEMENT | | | | 1011 (100)| | | | | | 1 | 5 | PX COORDINATOR | | | | | | | | | PLAN_TABLE_OUTPUT ------------------------------------------------------------------------------------------------------------------------------------------------------ | 2 | 4 | PX SEND QC (RANDOM) | :TQ10000 | 1391K| 33M| 1011 (1)| 00:00:13 | Q1,00 | P->S | QC (RAND) | | 3 | 3 | UPDATE | T1 | | | | | Q1,00 | PCWP | | | 4 | 2 | PX BLOCK ITERATOR | | 1391K| 33M| 1011 (1)| 00:00:13 | Q1,00 | PCWC | | |* 5 | 1 | TABLE ACCESS FULL| T1 | 1391K| 33M| 1011 (1)| 00:00:13 | Q1,00 | PCWP | | ----------------------------------------------------------------------------------------------------------------------- Query Block Name / Object Alias (identified by operation id): --------------------------------------------------------------------- 1 - UPD$1 5 - UPD$1 / A@UPD$1 PLAN_TABLE_OUTPUT ------------------------------------------------------------------------------------------------------------------------------------------------------ Outline Data ------------- /*+ BEGIN_OUTLINE_DATA IGNORE_OPTIM_EMBEDDED_HINTS OPTIMIZER_FEATURES_ENABLE('11.2.0.4') DB_VERSION('11.2.0.4') ALL_ROWS OUTLINE_LEAF(@"UPD$1") PLAN_TABLE_OUTPUT ------------------------------------------------------------------------------------------------------------------------------------------------------ FULL(@"UPD$1" "A"@"UPD$1") END_OUTLINE_DATA */ Predicate Information (identified by operation id): --------------------------------------------------- 5 - access(:Z>=:Z AND :Z<=:Z) Column Projection Information (identified by operation id): ----------------------------------------------------------- PLAN_TABLE_OUTPUT ------------------------------------------------------------------------------------------------------------------------------------------------------ 1 - SYSDEF[4], SYSDEF[32720], SYSDEF[1], SYSDEF[96], SYSDEF[32720] 2 - (#keys=0) SYSDEF[4], SYSDEF[32720], SYSDEF[1], SYSDEF[96], SYSDEF[32720] 3 - SYSDEF[4], SYSDEF[32720], SYSDEF[1], SYSDEF[96], SYSDEF[32720] 4 - (upd=2) "A".ROWID[ROWID,10], "OBJECT_NAME"[VARCHAR2,128] 5 - "A".ROWID[ROWID,10], "OBJECT_NAME"[VARCHAR2,128] 51 rows selected.