惜分飞

原sql语句与执行计划

SQL> set autot trace
SQL> WITH AL AS (SELECT * FROM XIFENFEI_LOG WHERE CLEAR_TIME IS NULL) 
   2 SELECT SWP.ID SWP_ID, AL.* FROM AL FULL OUTER JOIN XIFENFEI_LOG_SWAP SWP ON SWP.ID = AL.ID;

54 rows selected.


Execution Plan
----------------------------------------------------------
Plan hash value: 888046630

----------------------------------------------------------------------------------------------------------
| Id  | Operation                  | Name                        | Rows  | Bytes | Cost (%CPU)| Time     |
----------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT           |                             |    24 | 11064 | 24658   (2)| 00:04:56 |
|   1 |  TEMP TABLE TRANSFORMATION |                             |       |       |            |     |
|   2 |   LOAD AS SELECT           |                             |       |       |            |     |
|*  3 |    TABLE ACCESS FULL       | XIFENFEI_LOG                |    23 |  2576 | 24652   (2)| 00:04:56 |
|   4 |   VIEW                     |                             |    24 | 11064 |     6  (17)| 00:00:01 |
|   5 |    UNION-ALL               |                             |       |       |            |     |
|   6 |     NESTED LOOPS OUTER     |                             |    23 | 10465 |     2   (0)| 00:00:01 |
|   7 |      VIEW                  |                             |    23 | 10304 |     2   (0)| 00:00:01 |
|   8 |       TABLE ACCESS FULL    | SYS_TEMP_0FD9D6605_51B4E691 |    23 |  2576 |     2   (0)| 00:00:01 |
|*  9 |      INDEX UNIQUE SCAN     | XIFENFEI_LOG_SWP_PK         |     1 |     7 |     0   (0)| 00:00:01 |
|* 10 |     HASH JOIN ANTI         |                             |     1 |    20 |     4  (25)| 00:00:01 |
|  11 |      INDEX FULL SCAN       | XIFENFEI_LOG_SWP_PK         |    20 |   140 |     1   (0)| 00:00:01 |
|  12 |      VIEW                  |                             |    23 |   299 |     2   (0)| 00:00:01 |
|  13 |       TABLE ACCESS FULL    | SYS_TEMP_0FD9D6605_51B4E691 |    23 |  2576 |     2   (0)| 00:00:01 |
----------------------------------------------------------------------------------------------------------


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

   3 - filter("CLEAR_TIME" IS NULL)
   9 - access("SWP"."ID"(+)="AL"."ID")
  10 - access("SWP"."ID"="AL"."ID")


Statistics
----------------------------------------------------------
          2  recursive calls
          8  db block gets
     111504  consistent gets
          1  physical reads
        692  redo size
       8075  bytes sent via SQL*Net to client
        502  bytes received via SQL*Net from client
          5  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
         54  rows processed

这里很明显占用资源多，执行时间长的都在XIFENFEI_LOG表的全表扫描上,而该表的where 条件是CLEAR_TIME is null.


分析CLEAR_TIME 列null值的分布

SQL> SELECT count(*) FROM XIFENFEI_LOG WHERE CLEAR_TIME IS NULL;

  COUNT(*)
----------
        48

SQL> SELECT count(*) FROM XIFENFEI_LOG WHERE CLEAR_TIME IS not NULL;

  COUNT(*)
----------
   6899211

通过这里分析可以知道,CLEAR_TIME is null的值非常少,如果能够创建一个index,取到CLEAR_TIME 列null的值,那效率将非常搞.但是有oracle index知识的人都知道,B树index是不包含null列,因此一般性index无法满足该需求.这里思考创建含常数的复合index,而且把CLEAR_TIME放在前面,因为后面的常数一定存在,因此CLEAR_TIME中含有null的记录也就包含在该复合index中.

创建含常数复合index

SQL> create index ind_XIFENFEI_LOG_null on XIFENFEI_LOG (CLEAR_TIME,0) online;

Index created.

再次查看执行计划

SQL> WITH AL AS (SELECT * FROM XIFENFEI_LOG WHERE CLEAR_TIME IS NULL) 
  2  SELECT SWP.ID SWP_ID, AL.* FROM AL FULL OUTER JOIN XIFENFEI_LOG_SWAP SWP ON SWP.ID = AL.ID;

50 rows selected.


Execution Plan
----------------------------------------------------------
Plan hash value: 2359331571

-------------------------------------------------------------------------------------------------------------
| Id  | Operation                     | Name                        | Rows  | Bytes | Cost (%CPU)| Time     |
-------------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT              |                             |    24 | 11064 |    25   (4)| 00:00:01 |
|   1 |  TEMP TABLE TRANSFORMATION    |                             |       |       |            |          |
|   2 |   LOAD AS SELECT              |                             |       |       |            |          |
|   3 |    TABLE ACCESS BY INDEX ROWID| XIFENFEI_LOG                |    23 |  2576 |    19   (0)| 00:00:01 |
|*  4 |     INDEX RANGE SCAN          | IND_XIFENFEI_LOG_NULL       |    23 |       |     3   (0)| 00:00:01 |
|   5 |   VIEW                        |                             |    24 | 11064 |     6  (17)| 00:00:01 |
|   6 |    UNION-ALL                  |                             |       |       |            |          |
|   7 |     NESTED LOOPS OUTER        |                             |    23 | 10465 |     2   (0)| 00:00:01 |
|   8 |      VIEW                     |                             |    23 | 10304 |     2   (0)| 00:00:01 |
|   9 |       TABLE ACCESS FULL       | SYS_TEMP_0FD9D660D_51B4E691 |    23 |  2576 |     2   (0)| 00:00:01 |
|* 10 |      INDEX UNIQUE SCAN        | XIFENFEI_LOG_SWP_PK         |     1 |     7 |     0   (0)| 00:00:01 |
|* 11 |     HASH JOIN ANTI            |                             |     1 |    20 |     4  (25)| 00:00:01 |
|  12 |      INDEX FULL SCAN          | XIFENFEI_LOG_SWP_PK         |    20 |   140 |     1   (0)| 00:00:01 |
|  13 |      VIEW                     |                             |    23 |   299 |     2   (0)| 00:00:01 |
|  14 |       TABLE ACCESS FULL       | SYS_TEMP_0FD9D660D_51B4E691 |    23 |  2576 |     2   (0)| 00:00:01 |
-------------------------------------------------------------------------------------------------------------

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

   4 - access("CLEAR_TIME" IS NULL)
  10 - access("SWP"."ID"(+)="AL"."ID")
  11 - access("SWP"."ID"="AL"."ID")


Statistics
----------------------------------------------------------
          2  recursive calls
          8  db block gets
         33  consistent gets
          1  physical reads
        648  redo size
       7688  bytes sent via SQL*Net to client
        502  bytes received via SQL*Net from client
          5  SQL*Net roundtrips to/from client
          0  sorts (memory)
          0  sorts (disk)
         50  rows processed

这里可以发现,该sql使用了创建的含常数的复合index,sql执行时间从4分56秒,提高到现在的1秒钟,逻辑读从当初的111504减小到现在的33,巧用含常数的复合索引使得sql执行效率极大提高.