一个SQL语句的优化

原始SQL如下：

[html] view
plaincopyprint?

select * from (

select t.zxid,t.gh,t.xm,t.bm,t.fzjgdm,

(select count(a.session_id) from test_v a where to_char(t.zxid) = a.ZCRYZH) slzl,

(select count(a.session_id) from test_v a where to_char(t.zxid) = a.ZCRYZH and a.myd='0') 无评价,

(select count(a.session_id) from test_v a where to_char(t.zxid) = a.ZCRYZH and a.myd='1') 满意,

(select count(a.session_id) from test_v a where to_char(t.zxid) = a.ZCRYZH and a.myd='2') 较满意,

(select count(a.session_id) from test_v a where to_char(t.zxid) =&n
1fa47
bsp;a.ZCRYZH and a.myd='3') 一般,

(select count(a.session_id) from test_v a where to_char(t.zxid) = a.ZCRYZH and a.myd='4') 较不满意,

(select count(a.session_id) from test_v a where to_char(t.zxid) = a.ZCRYZH and a.myd='5') 不满意

from xxx t

WHERE t.yxbz='Y'

)where slzl<>0 ;

不用看执行计划就知道，表test_v会被扫描多次，所以这个SQL必须改写。如何改写这个SQL呢？请看下面分析：
首先，这个SQL语句是拼出来的，我们可以把复杂的问题简单化，所以我先看这个SQL：

[sql] view
plaincopyprint?

select t.zxid,t.gh,t.xm,t.bm,t.fzjgdm,

(select count(a.session_id) from test_v a where to_char(t.zxid) = a.ZCRYZH) slzl

from CC_ZXJBXX t

WHERE t.yxbz='Y';

执行计划如下：

[sql] view
plaincopyprint?

SQL> select t.zxid,t.gh,t.xm,t.bm,t.fzjgdm,

2 (select count(a.session_id) from test_v a where to_char(t.zxid) = a.ZCRYZH) slzl

3 from CC_ZXJBXX t

4 WHERE t.yxbz='Y';

已选择1956行。

已用时间: 00: 00: 00.37

执行计划

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

Plan hash value: 4286326665

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

| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |

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

| 0 | SELECT STATEMENT | | 1956 | 105K| 11 (0)| 00:00:01 |

| 1 | SORT AGGREGATE | | 1 | 22 | | |

|* 2 | TABLE ACCESS FULL| TEST_V | 4 | 88 | 5 (0)| 00:00:01 |

|* 3 | TABLE ACCESS FULL | CC_ZXJBXX | 1956 | 105K| 11 (0)| 00:00:01 |

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

Predicate Information (identified by operation id):

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

2 - filter("A"."ZCRYZH"=TO_CHAR(:B1))

3 - filter("T"."YXBZ"='Y')

上面的SQL语句会返回1956行，根据执行计划可以看到它进行的是filter操作，讲上面的SQL改写，初步改写如下：

[sql] view
plaincopyprint?

SQL> select t.zxid,t.gh,t.xm,t.bm,t.fzjgdm,count(*) from

2 CC_ZXJBXX t,test_v a where to_char(t.zxid) = a.ZCRYZH

3 and t.yxbz='Y' group by t.zxid,t.gh,t.xm,t.bm,t.fzjgdm;

已选择20行。

已用时间: 00: 00: 00.03

执行计划

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

Plan hash value: 2833546679

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

| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |

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

| 0 | SELECT STATEMENT | | 32157 | 2418K| 22 (28)| 00:00:01 |

| 1 | HASH GROUP BY | | 32157 | 2418K| 22 (28)| 00:00:01 |

|* 2 | HASH JOIN | | 32157 | 2418K| 17 (6)| 00:00:01 |

| 3 | TABLE ACCESS FULL| TEST_V | 411 | 9042 | 5 (0)| 00:00:01 |

|* 4 | TABLE ACCESS FULL| CC_ZXJBXX | 1956 | 105K| 11 (0)| 00:00:01 |

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

Predicate Information (identified by operation id):

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

2 - access("A"."ZCRYZH"=TO_CHAR("T"."ZXID"))

4 - filter("T"."YXBZ"='Y')

改写之后，SQL只返回了20条数据，但是原始SQL要返回1956条数据，问题出在哪里呢？ 问题在于，第一个SQL是filter，它是过滤，并且是count过滤
而第二个SQL是等值jion，第一个SQL里面的count 不管你过滤成功与否，都会返回数据，过滤失败，count=0 即可，所以对第二个SQL应该用 left join 于是SQL 更改如下：

[sql] view
plaincopyprint?

SQL> select t.zxid,t.gh,t.xm,t.bm,t.fzjgdm,count(*) from

2 CC_ZXJBXX t,test_v a where to_char(t.zxid) = a.ZCRYZH(+)

3 and t.yxbz='Y' group by t.zxid,t.gh,t.xm,t.bm,t.fzjgdm;

已选择1956行。

已用时间: 00: 00: 00.12

执行计划

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

Plan hash value: 3281235561

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

| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |

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

| 0 | SELECT STATEMENT | | 32157 | 2418K| 22 (28)| 00:00:01 |

| 1 | HASH GROUP BY | | 32157 | 2418K| 22 (28)| 00:00:01 |

|* 2 | HASH JOIN RIGHT OUTER| | 32157 | 2418K| 17 (6)| 00:00:01 |

| 3 | TABLE ACCESS FULL | TEST_V | 411 | 9042 | 5 (0)| 00:00:01 |

|* 4 | TABLE ACCESS FULL | CC_ZXJBXX | 1956 | 105K| 11 (0)| 00:00:01 |

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

Predicate Information (identified by operation id):

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

2 - access("A"."ZCRYZH"(+)=TO_CHAR("T"."ZXID"))

4 - filter("T"."YXBZ"='Y')

这样改写之后，返回记录条数是对了，但是SQL逻辑有点点改变，第一个SQL是过滤的，count()如果不匹配会返回0，但是第二个SQL是 join的，如果不匹配，会返回1





所以SQL需要改写如下：

[sql] view
plaincopyprint?

select t.zxid,t.gh,t.xm,t.bm,t.fzjgdm,decode(count(*),1,0,count(*)) from

CC_ZXJBXX t,test_v a where to_char(t.zxid) = a.ZCRYZH(+)

and t.yxbz='Y' group by t.zxid,t.gh,t.xm,t.bm,t.fzjgdm order by 1;

改写完了一个，后面的就简单了，所以整体的SQL可以改写如下：

[sql] view
plaincopyprint?

select t.zxid,

t.gh,

t.xm,

t.bm,

t.fzjgdm,

decode(count(*), 1, 0, count(*)),

SUM(DECODE(A.MYD, '0', 1, 0)) 无评价,

SUM(DECODE(A.MYD, '1', 1, 0)) 满意,

SUM(DECODE(A.MYD, '2', 1, 0)) 较满意,

SUM(DECODE(A.MYD, '3', 1, 0)) 一般,

SUM(DECODE(A.MYD, '4', 1, 0)) 较不满意,

SUM(DECODE(A.MYD, '5', 1, 0)) 不满意

from CC_ZXJBXX t, test_v a

where to_char(t.zxid) = a.ZCRYZH(+)

and t.yxbz = 'Y'

group by t.zxid, t.gh, t.xm, t.bm, t.fzjgdm ;

上面的SQL等价于

[sql] view
plaincopyprint?

select t.zxid,t.gh,t.xm,t.bm,t.fzjgdm,

(select count(a.session_id) from test_v a where to_char(t.zxid) = a.ZCRYZH) slzl,

(select count(a.session_id) from test_v a where to_char(t.zxid) = a.ZCRYZH and a.myd='0') 无评价,

(select count(a.session_id) from test_v a where to_char(t.zxid) = a.ZCRYZH and a.myd='1') 满意,

(select count(a.session_id) from test_v a where to_char(t.zxid) = a.ZCRYZH and a.myd='2') 较满意,

(select count(a.session_id) from test_v a where to_char(t.zxid) = a.ZCRYZH and a.myd='3') 一般,

(select count(a.session_id) from test_v a where to_char(t.zxid) = a.ZCRYZH and a.myd='4') 较不满意,

(select count(a.session_id) from test_v a where to_char(t.zxid) = a.ZCRYZH and a.myd='5') 不满意

from xxx t

WHERE t.yxbz='Y'；

由于原始SQL要去掉 slzl <>0 所以再包装一下

[sql] view
plaincopyprint?

select * from

(select t.zxid,

t.gh,

t.xm,

t.bm,

t.fzjgdm,

decode(count(*), 1, 0, count(*))slzl,

SUM(DECODE(A.MYD, '0', 1, 0)) 无评价,

SUM(DECODE(A.MYD, '1', 1, 0)) 满意,

SUM(DECODE(A.MYD, '2', 1, 0)) 较满意,

SUM(DECODE(A.MYD, '3', 1, 0)) 一般,

SUM(DECODE(A.MYD, '4', 1, 0)) 较不满意,

SUM(DECODE(A.MYD, '5', 1, 0)) 不满意

from CC_ZXJBXX t, test_v a

where to_char(t.zxid) = a.ZCRYZH(+)

and t.yxbz = 'Y'

group by t.zxid, t.gh, t.xm, t.bm, t.fzjgdm

) where slzl<>0;

原始SQL的执行计划 以及 更改后的执行计划对比：

[sql] view
plaincopyprint?

SQL> select * from (

2 select t.zxid,t.gh,t.xm,t.bm,t.fzjgdm,

3 (select count(a.session_id) from test_v a where to_char(t.zxid) = a.ZCRYZH) slzl,

4 (select count(a.session_id) from test_v a where to_char(t.zxid) = a.ZCRYZH and a.myd='0') 无评价,

5 (select count(a.session_id) from test_v a where to_char(t.zxid) = a.ZCRYZH and a.myd='1') 满意,

6 (select count(a.session_id) from test_v a where to_char(t.zxid) = a.ZCRYZH and a.myd='2') 较满意,

7 (select count(a.session_id) from test_v a where to_char(t.zxid) = a.ZCRYZH and a.myd='3') 一般,

8 (select count(a.session_id) from test_v a where to_char(t.zxid) = a.ZCRYZH and a.myd='4') 较不满意,

9 (select count(a.session_id) from test_v a where to_char(t.zxid) = a.ZCRYZH and a.myd='5') 不满意

10 from xxx t

11 WHERE t.yxbz='Y'

12 ) where slzl<>0;

已选择20行。

已用时间: 00: 00: 00.28

执行计划

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

Plan hash value: 2669170809

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

| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |

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

| 0 | SELECT STATEMENT | | 1956 | 76284 | 17 (0)| 00:00:01 |

| 1 | SORT AGGREGATE | | 1 | 4 | | |

|* 2 | TABLE ACCESS FULL | TEST_V | 16 | 64 | 5 (0)| 00:00:01 |

| 3 | SORT AGGREGATE | | 1 | 6 | | |

|* 4 | TABLE ACCESS FULL | TEST_V | 9 | 54 | 5 (0)| 00:00:01 |

| 5 | SORT AGGREGATE | | 1 | 6 | | |

|* 6 | TABLE ACCESS FULL | TEST_V | 5 | 30 | 5 (0)| 00:00:01 |

| 7 | SORT AGGREGATE | | 1 | 6 | | |

|* 8 | TABLE ACCESS FULL | TEST_V | 1 | 6 | 5 (0)| 00:00:01 |

| 9 | SORT AGGREGATE | | 1 | 6 | | |

|* 10 | TABLE ACCESS FULL | TEST_V | 2 | 12 | 5 (0)| 00:00:01 |

| 11 | SORT AGGREGATE | | 1 | 6 | | |

|* 12 | TABLE ACCESS FULL | TEST_V | 1 | 6 | 5 (0)| 00:00:01 |

| 13 | SORT AGGREGATE | | 1 | 6 | | |

|* 14 | TABLE ACCESS FULL | TEST_V | 1 | 6 | 5 (0)| 00:00:01 |

|* 15 | FILTER | | | | | |

|* 16 | TABLE ACCESS FULL | XXX | 1956 | 76284 | 12 (0)| 00:00:01 |

| 17 | SORT AGGREGATE | | 1 | 4 | | |

|* 18 | TABLE ACCESS FULL| TEST_V | 16 | 64 | 5 (0)| 00:00:01 |

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

Predicate Information (identified by operation id):

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

2 - filter("A"."ZCRYZH"=TO_CHAR(:B1))

4 - filter("A"."ZCRYZH"=TO_CHAR(:B1) AND "A"."MYD"='0')

6 - filter("A"."ZCRYZH"=TO_CHAR(:B1) AND "A"."MYD"='1')

8 - filter("A"."ZCRYZH"=TO_CHAR(:B1) AND "A"."MYD"='2')

10 - filter("A"."ZCRYZH"=TO_CHAR(:B1) AND "A"."MYD"='3')

12 - filter("A"."ZCRYZH"=TO_CHAR(:B1) AND "A"."MYD"='4')

14 - filter("A"."ZCRYZH"=TO_CHAR(:B1) AND "A"."MYD"='5')

15 - filter( (SELECT COUNT(*) FROM "TEST_V" "A" WHERE

"A"."ZCRYZH"=TO_CHAR(:B1))<>0)

16 - filter("T"."YXBZ"='Y')

18 - filter("A"."ZCRYZH"=TO_CHAR(:B1))

统计信息

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

1 recursive calls

0 db block gets

33261 consistent gets

0 physical reads

0 redo size

2011 bytes sent via SQL*Net to client

411 bytes received via SQL*Net from client

3 SQL*Net roundtrips to/from client

0 sorts (memory)

0 sorts (disk)

20 rows processed

原始SQL需要消耗33261个逻辑读，扫描TEST_V表8次，更改之后的SQL

[sql] view
plaincopyprint?

SQL> select * from

2 (select t.zxid,

3 t.gh,

4 t.xm,

5 t.bm,

6 t.fzjgdm,

7 decode(count(*), 1, 0, count(*))slzl,

8 SUM(DECODE(A.MYD, '0', 1, 0)) 无评价,

9 SUM(DECODE(A.MYD, '1', 1, 0)) 满意,

10 SUM(DECODE(A.MYD, '2', 1, 0)) 较满意,

11 SUM(DECODE(A.MYD, '3', 1, 0)) 一般,

12 SUM(DECODE(A.MYD, '4', 1, 0)) 较不满意,

13 SUM(DECODE(A.MYD, '5', 1, 0)) 不满意

14 from CC_ZXJBXX t, test_v a

15 where to_char(t.zxid) = a.ZCRYZH(+)

16 and t.yxbz = 'Y'

17 group by t.zxid, t.gh, t.xm, t.bm, t.fzjgdm

18 ) where slzl<>0;

已选择20行。

已用时间: 00: 00: 00.01

执行计划

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

Plan hash value: 1348291491

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

| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |

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

| 0 | SELECT STATEMENT | | 98 | 4410 | 18 (12)| 00:00:01 |

|* 1 | FILTER | | | | | |

| 2 | HASH GROUP BY | | 98 | 4410 | 18 (12)| 00:00:01 |

|* 3 | HASH JOIN RIGHT OUTER| | 1956 | 88020 | 17 (6)| 00:00:01 |

| 4 | TABLE ACCESS FULL | TEST_V | 411 | 2466 | 5 (0)| 00:00:01 |

|* 5 | TABLE ACCESS FULL | CC_ZXJBXX | 1956 | 76284 | 11 (0)| 00:00:01 |

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

Predicate Information (identified by operation id):

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

1 - filter(DECODE(COUNT(*),1,0,COUNT(*))<>0)

3 - access("A"."ZCRYZH"(+)=TO_CHAR("T"."ZXID"))

5 - filter("T"."YXBZ"='Y')

统计信息

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

0 recursive calls

0 db block gets

62 consistent gets

0 physical reads

0 redo size

1958 bytes sent via SQL*Net to client

411 bytes received via SQL*Net from client

3 SQL*Net roundtrips to/from client

0 sorts (memory)

0 sorts (disk)

20 rows processed

逻辑读降低到62，表TEST_V只扫描1次，其实这个SQL只是一个汇总的SQL，稍微有点开发常识的人都知道，最开始写SQL的那个人绝对是在乱写SQL。SQL的优化到这里还没完，
因为我们已经知道这个SQL的目的其实就是汇总，所以直接抛弃刚才的优化思路，直接重写SQL如下：

[sql] view
plaincopyprint?

SQL> SELECT *

2 FROM (SELECT T.ZXID,

3 T.GH,

4 T.XM,

5 T.BM,

6 T.FZJGDM,

7 SUM(1) SLZL,

8 SUM(DECODE(A.MYD, '0', 1, 0)) 无评价,

9 SUM(DECODE(A.MYD, '1', 1, 0)) 满意,

10 SUM(DECODE(A.MYD, '2', 1, 0)) 较满意,

11 SUM(DECODE(A.MYD, '3', 1, 0)) 一般,

12 SUM(DECODE(A.MYD, '4', 1, 0)) 较不满意,

13 SUM(DECODE(A.MYD, '5', 1, 0)) 不满意

14 FROM CC_ZXJBXX T ,test_v A

15 where A.ZCRYZH=T.ZXID

16 and T.YXBZ = 'Y'

17 GROUP BY T.ZXID, T.GH, T.XM, T.BM, T.FZJGDM) order by 1;

已选择20行。

已用时间: 00: 00: 00.01

执行计划

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

Plan hash value: 1317439664

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

| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |

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

| 0 | SELECT STATEMENT | | 411 | 18495 | 18 (12)| 00:00:01 |

| 1 | SORT GROUP BY | | 411 | 18495 | 18 (12)| 00:00:01 |

|* 2 | HASH JOIN | | 411 | 18495 | 17 (6)| 00:00:01 |

| 3 | TABLE ACCESS FULL| TEST_V | 411 | 2466 | 5 (0)| 00:00:01 |

|* 4 | TABLE ACCESS FULL| CC_ZXJBXX | 1956 | 76284 | 11 (0)| 00:00:01 |

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

Predicate Information (identified by operation id):

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

2 - access("T"."ZXID"=TO_NUMBER("A"."ZCRYZH"))

4 - filter("T"."YXBZ"='Y')

统计信息

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

0 recursive calls

0 db block gets

62 consistent gets

0 physical reads

0 redo size

1945 bytes sent via SQL*Net to client

411 bytes received via SQL*Net from client

3 SQL*Net roundtrips to/from client

1 sorts (memory)

0 sorts (disk)

20 rows processed

有时候开发人员，为了完成任务，他才不管你SQL写得好不好呢，能完成功能就行了，相信各位DBA深有体会。