优化实例- not in 和 not exists

客户运行一个SQL，非常慢。于是进行了一下改写。速度飞快，首先看一下原来的SQL。

original sql

SQL> explain plan for
  2  select count(*) from pnadmin.si_vsl where vsl_status_i = 'A' and to_number(vsl_id_n) not in (select vessel_id from pnadmin.vessel_master);

Explained.

SQL> select * from table(dbms_xplan.display());

PLAN_TABLE_OUTPUT
-------------------------------------------------------------------------------------------------------------------------------------------------------------------
Plan hash value: 3613440888

-------------------------------------------------------------------------------------
| Id  | Operation           | Name          | Rows  | Bytes | Cost (%CPU)| Time     |
-------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT    |               |     1 |     9 |  9466K  (4)| 31:33:15 |
|   1 |  SORT AGGREGATE     |               |     1 |     9 |            |          |
|*  2 |   FILTER            |               |       |       |            |          |
|*  3 |    TABLE ACCESS FULL| SI_VSL        | 35069 |   308K|   642   (5)| 00:00:08 |
|*  4 |    TABLE ACCESS FULL| VESSEL_MASTER |     1 |     5 |   352   (4)| 00:00:05 |
-------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   2 - filter( NOT EXISTS (SELECT 0 FROM "PNADMIN"."VESSEL_MASTER"
              "VESSEL_MASTER" WHERE LNNVL("VESSEL_ID"<>TO_NUMBER(:B1))))
   3 - filter("VSL_STATUS_I"='A')
   4 - filter(LNNVL("VESSEL_ID"<>TO_NUMBER(:B1)))

19 rows selected.

　　

可以看到是filter 操作。 filter操作在之前的blog中有介绍（http://www.cnblogs.com/kramer/archive/2013/04/12/3017013.html）。 这是一个类似于nested loop的操作，可以理解为对SI_VSL的每一行都要对VESSEL_MASTER做一次全表扫描，所以这个很显然是会非常慢的。 于是想到如果用hash join可能会快很多。 两种办法，一种是改写，一种是用 hash hint。 我用的是改写

new sql

SQL> explain plan for
  2  select count(*) from PNADMIN.si_vsl a where a.vsl_status_i = 'A' and not exists (select 1 from PNADMIN.vessel_master b where b.vessel_id=to_number(a.vsl_id_n));

Explained.

SQL> select * from table(dbms_xplan.display());

PLAN_TABLE_OUTPUT
-------------------------------------------------------------------------------------------------------------------------------------------------------------------
Plan hash value: 3198032547

-------------------------------------------------------------------------------------
| Id  | Operation           | Name          | Rows  | Bytes | Cost (%CPU)| Time     |
-------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT    |               |     1 |    14 |   995   (4)| 00:00:12 |
|   1 |  SORT AGGREGATE     |               |     1 |    14 |            |          |
|*  2 |   HASH JOIN ANTI    |               | 35067 |   479K|   995   (4)| 00:00:12 |
|*  3 |    TABLE ACCESS FULL| SI_VSL        | 35069 |   308K|   641   (5)| 00:00:08 |
|   4 |    TABLE ACCESS FULL| VESSEL_MASTER | 74163 |   362K|   348   (3)| 00:00:05 |
-------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   2 - access("B"."VESSEL_ID"=TO_NUMBER("A"."VSL_ID_N"))
   3 - filter("A"."VSL_STATUS_I"='A')

　　

新的SQL用hash 非常快。 但是我想通过hash hint的方式修改原sql 效果不好。 也许是因为对hash hint理解不深，明天好好研究一下。

--

后来发现 hash_aj hint在 oracle10g中不支持了。 同样的还有很多

The AND_EQUAL, HASH_AJ, MERGE_AJ, NL_AJ, HASH_SJ, MERGE_SJ, NL_SJ, EXPAND_GSET_TO_UNION, ORDERED_PREDICATES, ROWID, and STAR hints have been deprecated and should not be used. 详细请看下面的URL

http://docs.oracle.com/cd/B13789_01/server.101/b10752/whatsnew.htm

OK 既然hash hint不支持了 我们就不再纠结了 。  本文只要记得 至少在10g里面 通过 not in exist 等的转换还可以获得不同的执行计划就好了。