oracle中如何改写exists降低逻辑读

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通过将exists改写成in或这inner join优化sql。

Sql_id：056bs9dzz8mwy

问题简述：逻辑读高。

Sql文本：

SELECT A.*, a.rowid

FROM WBANK.WD_BANK_BASEINFOMATION A

WHERE EXISTS (SELECT 1

FROM (select KEYWORD, TYPECODE, INNERCODE, COUNT(*)

FROM WBANK.WD_BANK_BASEINFOMATION

WHERE SUBSTR(TYPECODE, 1, 3) = '001'

GROUP BY KEYWORD, TYPECODE, INNERCODE

HAVING COUNT(*) <> 1) B

WHERE A.KEYWORD = B.KEYWORD

AND A.TYPECODE = B.TYPECODE

AND A.INNERCODE = B.INNERCODE);

执行计划：

Execution Plan

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

Plan hash value: 1318914978

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

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

| 0 | SELECT STATEMENT | | 1 | 130 | 7930K (1)| 39:39:10 |

|* 1 | FILTER | | | | | |

| 2 | TABLE ACCESS FULL | WD_BANK_BASEINFOMATION | 2640K| 327M| 6249 (2)| 00:01:53 |

|* 3 | FILTER | | | | | |

| 4 | SORT GROUP BY NOSORT| | 1 | 47 | 3 (0)| 00:00:01 |

|* 5 | INDEX RANGE SCAN | IDX_WD_B_BI | 1 | 47 | 3 (0)| 00:00:01 |

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

Predicate Information (identified by operation id):

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

1 - filter( EXISTS (SELECT 0 FROM "WBANK"."WD_BANK_BASEINFOMATION"

"WD_BANK_BASEINFOMATION" WHERE "TYPECODE"=:B1 AND "KEYWORD"=:B2 AND "INNERCODE"=:B3 AND

SUBSTR("TYPECODE",1,3)='001' GROUP BY "KEYWORD","TYPECODE","INNERCODE" HAVING

COUNT(*)<>1))

3 - filter(COUNT(*)<>1)

5 - access("KEYWORD"=:B1 AND "TYPECODE"=:B2 AND "INNERCODE"=:B3)

filter("INNERCODE"=:B1 AND SUBSTR("TYPECODE",1,3)='001')

Statistics

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

1 recursive calls

0 db block gets

2329554 consistent gets

13 physical reads

0 redo size

2507 bytes sent via SQL*Net to client

513 bytes received via SQL*Net from client

1 SQL*Net roundtrips to/from client

0 sorts (memory)

0 sorts (disk)

0 rows processed

可以发现逻辑读高达200多万。

刚看到这个sql的时候猜想会不会逻辑有问题，导致结果集为空。跑了一遍发现结果集确实为空。子查询的innercode列全部为null。根据条件A.INNERCODE = B.INNERCODE外部表（虽然是同一张表）是不会有匹配结果的。转念一想如果子查询innercode列有非空的，那就不会有问题了。当然了还是要询问开发结果集与该列为空是否有必然联系，如果有联系的话可以利用该逻辑关系改写sql。当然，这是后话了。

看一下数据分布：

SQL> select count(*) from WBANK.WD_BANK_BASEINFOMATION;

COUNT(*)

----------

2645546

SQL> select count(*) from (select KEYWORD, TYPECODE, INNERCODE, COUNT(*)

2 FROM WBANK.WD_BANK_BASEINFOMATION

3 WHERE SUBSTR(TYPECODE, 1, 3) = '001'

4 GROUP BY KEYWORD, TYPECODE, INNERCODE

5 HAVING COUNT(*) <> 1);

COUNT(*)

----------

128

外层结果集是全表数据260多万。子查询结果集只有128条。而根据oracle对exists的处理，会以外部结果集为驱动，也就是说要执行260多万次，这显然是不合理的。如果外部结果集大，内部结果集小的话，这种情况下通常是要用in，以内部结果集为驱动，这样也就执行128次。

验证一下执行次数的问题：

SQL> alter session set statistics_level=all;

SQL> SELECT A.*, a.rowid

2 FROM WBANK.WD_BANK_BASEINFOMATION A

3 WHERE EXISTS (SELECT 1

4 FROM (select KEYWORD, TYPECODE, INNERCODE, COUNT(*)

5 FROM WBANK.WD_BANK_BASEINFOMATION

6 WHERE SUBSTR(TYPECODE, 1, 3) = '001'

7 GROUP BY KEYWORD, TYPECODE, INNERCODE

8 HAVING COUNT(*) <> 1) B

9 WHERE A.KEYWORD = B.KEYWORD

10 AND A.TYPECODE = B.TYPECODE

11 AND A.INNERCODE = B.INNERCODE);

no rows selected

SQL> SELECT * FROM TABLE(dbms_xplan.display_cursor(null,null,'ALLSTATS LAST'));

Plan hash value: 1318914978

PLAN_TABLE_OUTPUT

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

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

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

| 0 | SELECT STATEMENT | | 1 | | 0 |00:00:09.75 | 2329K|

|* 1 | FILTER | | 1 | | 0 |00:00:09.75 | 2329K|

| 2 | TABLE ACCESS FULL | WD_BANK_BASEINFOMATION | 1 | 2640K| 2645K|00:00:00.61 | 12226 |

|* 3 | FILTER | | 2632K| | 0 |00:00:07.38 | 2317K|

| 4 | SORT GROUP BY NOSORT| | 2632K| 1 | 1273K|00:00:06.64 | 2317K|

|* 5 | INDEX RANGE SCAN | IDX_WD_B_BI | 2632K| 1 | 1273K|00:00:03.42 | 2317K|

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

PLAN_TABLE_OUTPUT

Predicate Information (identified by operation id):

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

1 - filter( IS NOT NULL)

3 - filter(COUNT(*)<>1)

5 - access("KEYWORD"=:B1 AND "TYPECODE"=:B2 AND "INNERCODE"=:B3)

filter(("INNERCODE"=:B1 AND SUBSTR("TYPECODE",1,3)='001'))

31 rows selected.

可以看到starts列部分，内部子查询2632k次，与外表数据量吻合。

用in改写sql

SELECT A.*, a.rowid

FROM WBANK.WD_BANK_BASEINFOMATION A

WHERE (A.KEYWORD,A.TYPECODE,A.INNERCODE) in (SELECT B.KEYWORD,B.TYPECODE,B.INNERCODE

FROM (select KEYWORD, TYPECODE, INNERCODE, COUNT(*)

FROM WBANK.WD_BANK_BASEINFOMATION

WHERE SUBSTR(TYPECODE, 1, 3) = '001'

GROUP BY KEYWORD, TYPECODE, INNERCODE

HAVING COUNT(*) <> 1) B

);

执行计划：

Set autotrace on

执行sql。

得到执行计划：

Execution Plan

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

Plan hash value: 1385212545

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

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

| 0 | SELECT STATEMENT | | 3 | 7008 | 6236 (2)| 00:01:53 |

| 1 | NESTED LOOPS | | 3 | 7008 | 6236 (2)| 00:01:53 |

| 2 | NESTED LOOPS | | 3 | 7008 | 6236 (2)| 00:01:53 |

| 3 | VIEW | VW_NSO_1 | 55 | 118K| 6228 (2)| 00:01:53 |

|* 4 | FILTER | | | | | |

| 5 | HASH GROUP BY | | 1 | 2585 | 6228 (2)| 00:01:53 |

|* 6 | TABLE ACCESS FULL | WD_BANK_BASEINFOMATION | 26410 | 1212K| 6226 (2)| 00:01:53 |

|* 7 | INDEX RANGE SCAN | IDX_WD_B_BI | 1 | | 2 (0)| 00:00:01 |

| 8 | TABLE ACCESS BY INDEX ROWID| WD_BANK_BASEINFOMATION | 1 | 130 | 3 (0)| 00:00:01 |

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

Predicate Information (identified by operation id):

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

4 - filter(COUNT(*)<>1)

6 - filter(SUBSTR("TYPECODE",1,3)='001')

7 - access("A"."KEYWORD"="KEYWORD" AND "A"."TYPECODE"="TYPECODE" AND

"A"."INNERCODE"="INNERCODE")

filter("A"."INNERCODE" IS NOT NULL AND "A"."INNERCODE"="INNERCODE")

Statistics

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

1 recursive calls

0 db block gets

12226 consistent gets

0 physical reads

0 redo size

2507 bytes sent via SQL*Net to client

513 bytes received via SQL*Net from client

1 SQL*Net roundtrips to/from client

0 sorts (memory)

0 sorts (disk)

0 rows processed

执行计划已经变成以内部子查询为驱动表了。而且逻辑读从200万降低1万。

下面再来验证执行次数：

SQL> alter session set statistics_level=all;

执行sql

SQL> SELECT * FROM TABLE(dbms_xplan.display_cursor(null,null,'ALLSTATS LAST'));

得到执行计划（部分）：

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

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

| 0 | SELECT STATEMENT | | 1 | | 0 |00:00:02.28 | 12226 | |

|* 1 | HASH JOIN RIGHT SEMI | | 1 | 1311K| 0 |00:00:02.28 | 12226 | 391K|

| 2 | VIEW | VW_NSO_1 | 1 | 80389 | 128 |00:00:02.28 | 12226 | |

|* 3 | FILTER | | 1 | | 128 |00:00:02.28 | 12226 | |

| 4 | HASH GROUP BY | | 1 | 4020 | 1607K|00:00:02.17 | 12226 | 710M|

|* 5 | TABLE ACCESS FULL| WD_BANK_BASEINFOMATION | 1 | 1607K| 1607K|00:00:00.78 | 12226 | |

|* 6 | TABLE ACCESS FULL | WD_BANK_BASEINFOMATION | 0 | 1311K| 0 |00:00:00.01 | 0 | |

发现执行计划并不一致，这个才是真正的执行计划。

Predicate Information (identified by operation id):

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

1 - access("A"."KEYWORD"="KEYWORD" AND "A"."TYPECODE"="TYPECODE" AND "A"."INNERCODE"="INNERCODE")

3 - filter(COUNT(*)<>1)

5 - filter(SUBSTR("TYPECODE",1,3)='001')

6 - filter("A"."INNERCODE" IS NOT NULL)

Note

PLAN_TABLE_OUTPUT

-----

- cardinality feedback used for this statement

后面发现了基数反馈的东西。估计值是实际值差别还是很大的。说明统计信息是有问题的。

查看统计信息已经是4月份收集的了。

收集统计信息

SQL> exec dbms_stats.gather_table_stats(ownname => 'WBANK',tabname => 'WD_BANK_BASEINFOMATION',estimate_percent => 10,method_opt=> 'for all columns size repeat',no_invalidate=>false);

PL/SQL procedure successfully completed.

收集完统计信息后的执行计划

Plan hash value: 1385212545

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

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

| 0 | SELECT STATEMENT | | 1 | | 2 |00:00:02.55 | 12232 | | | |

| 1 | NESTED LOOPS | | 1 | 3 | 2 |00:00:02.55 | 12232 | | | |

| 2 | NESTED LOOPS | | 1 | 3 | 2 |00:00:02.55 | 12230 | | | |

| 3 | VIEW | VW_NSO_1 | 1 | 53 | 129 |00:00:02.55 | 12226 | | | |

|* 4 | FILTER | | 1 | | 129 |00:00:02.55 | 12226 | | | |

| 5 | HASH GROUP BY | | 1 | 1 | 1607K|00:00:02.40 | 12226 | 710M| 17M| 170M (0)|

|* 6 | TABLE ACCESS FULL | WD_BANK_BASEINFOMATION | 1 | 26458 | 1607K|00:00:00.80 | 12226 | | | |

|* 7 | INDEX RANGE SCAN | IDX_WD_B_BI | 129 | 1 | 2 |00:00:00.01 | 4 | | | |

| 8 | TABLE ACCESS BY INDEX ROWID| WD_BANK_BASEINFOMATION | 2 | 1 | 2 |00:00:00.01 | 2 | | | |

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

Predicate Information (identified by operation id):

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

4 - filter(COUNT(*)<>1)

6 - filter(SUBSTR("TYPECODE",1,3)='001')

7 - access("A"."KEYWORD"="KEYWORD" AND "A"."TYPECODE"="TYPECODE" AND "A"."INNERCODE"="INNERCODE")

filter(("A"."INNERCODE" IS NOT NULL AND "A"."INNERCODE"="INNERCODE"))

34 rows selected.

可以看到确实是129次。而且也不存在基数反馈导致执行计划改变了。逻辑读还是在1万多。

突然想到还可以使用inner join的方法来改写sql

SELECT A.*, a.rowid

FROM WBANK.WD_BANK_BASEINFOMATION A

inner join (select KEYWORD, TYPECODE, INNERCODE, COUNT(*)

FROM WBANK.WD_BANK_BASEINFOMATION

WHERE SUBSTR(TYPECODE, 1, 3) = '001'

GROUP BY KEYWORD, TYPECODE, INNERCODE

HAVING COUNT(*) <> 1) B

on A.KEYWORD = B.KEYWORD

AND A.TYPECODE = B.TYPECODE

AND A.INNERCODE = B.INNERCODE;

执行计划：

Plan hash value: 4254729379

PLAN_TABLE_OUTPUT

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

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

| 0 | SELECT STATEMENT | | 1 | | 2 |00:00:02.48 | 12232 | | | |

| 1 | NESTED LOOPS | | 1 | 59 | 2 |00:00:02.48 | 12232 | | | |

| 2 | NESTED LOOPS | | 1 | 59 | 2 |00:00:02.48 | 12230 | | | |

| 3 | VIEW | | 1 | 59 | 129 |00:00:02.48 | 12226 | | | |

|* 4 | FILTER | | 1 | | 129 |00:00:02.48 | 12226 | | | |

| 5 | HASH GROUP BY | | 1 | 59 | 1607K|00:00:02.31 | 12226 | 710M| 17M| 168M (0)|

|* 6 | TABLE ACCESS FULL | WD_BANK_BASEINFOMATION | 1 | 26466 | 1607K|00:00:00.76 | 12226 | | | |

PLAN_TABLE_OUTPUT

|* 7 | INDEX RANGE SCAN | IDX_WD_B_BI | 129 | 1 | 2 |00:00:00.01 | 4 | | | |

| 8 | TABLE ACCESS BY INDEX ROWID| WD_BANK_BASEINFOMATION | 2 | 1 | 2 |00:00:00.01 | 2 | | | |

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

Predicate Information (identified by operation id):

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

4 - filter(COUNT(*)<>1)

6 - filter(SUBSTR("TYPECODE",1,3)='001')

7 - access("A"."KEYWORD"="B"."KEYWORD" AND "A"."TYPECODE"="B"."TYPECODE" AND "A"."INNERCODE"="B"."INNERCODE")

filter(("A"."INNERCODE" IS NOT NULL AND "A"."INNERCODE"="B"."INNERCODE"))

PLAN_TABLE_OUTPUT

34 rows selected.

逻辑读

Statistics

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

1 recursive calls

0 db block gets

12232 consistent gets

0 physical reads

0 redo size

3083 bytes sent via SQL*Net to client

524 bytes received via SQL*Net from client

2 SQL*Net roundtrips to/from client

0 sorts (memory)

0 sorts (disk)

2 rows processed

逻辑读也是1万多。

看一下执行计划发现，瓶颈都在对表的全表扫且过滤条件filter(SUBSTR("TYPECODE",1,3)='001')。

可以考虑在这列上建函数索引，

SQL> select count(*) from wbank.WD_BANK_BASEINFOMATION WHERE SUBSTR(TYPECODE, 1, 3) = '001';

COUNT(*)

----------

1607674

表的数据一共只有2645546，返回1607674，所以建了索引也没用，所以不用建索引了。

综上所述。优化建议是更改sql，将exists改成in或者inner join：

SELECT A.*, a.rowid

FROM WBANK.WD_BANK_BASEINFOMATION A

WHERE (A.KEYWORD,A.TYPECODE,A.INNERCODE) in (SELECT B.KEYWORD,B.TYPECODE,B.INNERCODE

FROM (select KEYWORD, TYPECODE, INNERCODE, COUNT(*)

FROM WBANK.WD_BANK_BASEINFOMATION

WHERE SUBSTR(TYPECODE, 1, 3) = '001'

GROUP BY KEYWORD, TYPECODE, INNERCODE

HAVING COUNT(*) <> 1) B

);

或者

SELECT A.*, a.rowid

FROM WBANK.WD_BANK_BASEINFOMATION A

inner join (select KEYWORD, TYPECODE, INNERCODE, COUNT(*)

FROM WBANK.WD_BANK_BASEINFOMATION

WHERE SUBSTR(TYPECODE, 1, 3) = '001'

GROUP BY KEYWORD, TYPECODE, INNERCODE

HAVING COUNT(*) <> 1) B

on A.KEYWORD = B.KEYWORD

AND A.TYPECODE = B.TYPECODE

AND A.INNERCODE = B.INNERCODE;

逻辑读将从200多万将至1万多。

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