一次fast full scan的调优

1.闲着无聊在客户一套较为重要的系统中提取了一个4天的AWR，发现下面这条SQL产生最多的物理读，四天运行了159次。平均每次物理读1G左右。SQL运行时间三分钟左右。

SQL:

SELECT DECODE(B.DH, 'MAIN', :1, B.DH) DH, COUNT(DISTINCT(A.PROCESSID)) VALUE,

B.MC COMPANYNAME FROM FMIS3000.WF_ACTIVITIES A, (SELECT DH, MC FROM FMIS3000.XTDW WHERE 1=1 )

B WHERE A.ACTTYPE = 1 AND TRIM(A.DH) = TRIM(B.DH) GROUP BY B.DH, B.MC;

2.我查看了执行计划：

PLAN_TABLE_OUTPUT

------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------

Plan hash value: 2953189885

---------------------------------------------------------------------------------------------

---------------------------------------------------------------------------------------------

| 0 | SELECT STATEMENT | | 191K| 18M| | 300K (1)| 01:10:12 |

| 1 | SORT GROUP BY | | 191K| 18M| 4006M| 300K (1)| 01:10:12 |

|* 2 | HASH JOIN | | 37M| 3516M| | 45650 (1)| 00:10:40 |

| 3 | TABLE ACCESS FULL| XTDW | 528 | 18480 | | 4 (0)| 00:00:01 |

|* 4 | TABLE ACCESS FULL| WF_ACTIVITIES | 7054K| 430M| | 45478 (1)| 00:10:37 |

---------------------------------------------------------------------------------------------

PLAN_TABLE_OUTPUT

Predicate Information (identified by operation id):

---------------------------------------------------

2 - access(TRIM("A"."DH")=TRIM("DH"))

4 - filter("A"."ACTTYPE"=1)

17 rows selected.

3.我注意到有两个全表扫描，于是我查找执行这个SQL时两个表会返回的数据量

这是一个HASH连接 FMIS3000.XTDW 表为驱动表，可以看到他数据量不大，作为驱动表还是很合适的，走table access full，也比较合理。

SQL> select count(*) from FMIS3000.XTDW ;

COUNT(*)

----------

530

4.这是被驱动表，数据量比较大。我发现这儿应是一个瓶颈。

SQL> select count(*) from FMIS3000.WF_ACTIVITIES where ACTTYPE=1;

COUNT(*)

----------

12078901

5.这个查询返回1.2KW数据，整个表有1.7KW数据。走全表扫描其实并没有错，但是我在想如何解决这个瓶颈

6.接着我返回去看SQL，发现我不需要扫描整个表的数据：

经过查询这个表有26列，我可以在需要的列上面创建索引。

create index id_tex on FMIS3000.WF_ACTIVITIES（ACTTYPE，TRIM(DH)，PROCESSID）；

在这儿我没有考虑选择性（因为不需要，而且选择性肯定奇差），我推测这样，可以走INDEX FAST FULL SCAN，且不用回表。将会有效提升效率。

建立索引前：

PLAN_TABLE_OUTPUT

SQL_ID 4n6ajf2fsm2x9, child number 0

-------------------------------------

SELECT DECODE(B.DH, 'MAIN','MAIN', B.DH) DH, COUNT(DISTINCT(A.PROCESSID)) VALUE, B.MC COMPANYNAME FROM

FMIS3000.WF_ACTIVITIES A, (SELECT DH, MC FROM FMIS3000.XTDW WHERE 1=1 ) B WHERE A.ACTTYPE = 1 AND

TRIM(A.DH) = TRIM(B.DH) GROUP BY B.DH, B.MC

Plan hash value: 2953189885

-----------------------------------------------------------------------------------------------------------------

-----------------------------------------------------------------------------------------------------------------

| 1 | SORT GROUP BY | | 1 | 191K| 475 |00:02:51.39 | 285K| 423K| 138K|

|* 2 | HASH JOIN | | 1 | 37M| 12M|00:01:12.56 | 285K| 285K| 0 |

| 3 | TABLE ACCESS FULL| XTDW | 1 | 528 | 530 |00:00:00.02 | 10 | 8 | 0 |

|* 4 | TABLE ACCESS FULL| WF_ACTIVITIES | 1 | 7054K| 12M|00:00:48.38 | 285K| 285K| 0 |

-----------------------------------------------------------------------------------------------------------------

Statistics

----------------------------------------------------------

1082 recursive calls

71 db block gets

285137 consistent gets

311743 physical reads

0 redo size

2208 bytes sent via SQL*Net to client

514 bytes received via SQL*Net from client

3 SQL*Net roundtrips to/from client

0 sorts (memory)

1 sorts (disk)

30 rows processed

建立索引后：

PLAN_TABLE_OUTPUT

SQL_ID g5pavm81dbpdt, child number 0

-------------------------------------

SELECT DECODE(B.DH, 'MAIN', 'MAIN', B.DH) DH, COUNT(DISTINCT(A.PROCESSID)) VALUE, B.MC COMPANYNAME

FROM FMIS3000.WF_ACTIVITIES A, (SELECT DH, MC FROM FMIS3000.XTDW WHERE 1=1 ) B WHERE A.ACTTYPE = 1

AND TRIM(A.DH) = TRIM(B.DH) GROUP BY B.DH, B.MC

Plan hash value: 513252325

----------------------------------------------------------------------------------------

----------------------------------------------------------------------------------------

| 0 | SELECT STATEMENT | | 191K| 18M| | 341K (2)| 01:08:17 |

| 1 | SORT GROUP BY | | 191K| 18M| 4062M| 341K (2)| 01:08:17 |

|* 2 | HASH JOIN | | 37M| 3516M| | 39892 (2)| 00:07:59 |

| 3 | TABLE ACCESS FULL | XTDW | 528 | 18480 | | 4 (0)| 00:00:01 |

|* 4 | INDEX FAST FULL SCAN| TE_DX | 7054K| 430M| | 39524 (2)| 00:07:55 |

----------------------------------------------------------------------------------------

Statistics

----------------------------------------------------------

1074 recursive calls

51 db block gets

179736 consistent gets

124315 physical reads

0 redo size

2208 bytes sent via SQL*Net to client

514 bytes received via SQL*Net from client

3 SQL*Net roundtrips to/from client

0 sorts (memory)

1 sorts (disk)

30 rows processed

这个执行计划是真实的

可以很明显的看到，不管是逻辑读和物理读都少了一倍左右。（我在执行前都flush了share pool和buffer cache）

建立索引之后 CBO自动选择了INDEX FAST FULL SCAN ，而且确实没有回表。和预估的结果一样。

总的执行时间减少了一倍，在扫描WF_ACTIVITIES表阶段时间从48秒减少到了8秒。性能提升了6倍。

但是这个SQL还有个瓶颈是SORT GROUP BY 也就是DISTINCT排序导致的。我也试验了，如果不要distinct，执行时间还可以减少30多秒，这个和业务相关了，就没有太大办法，而且被驱动表返回结果1.2KW行，hash不可能不消耗时间，这个确实没办法。如果知道业务或许可以改写SQL。

一次fast full scan的调优

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