在Oracle中，和子查询相关的查询转换有哪些？
♣ 答案部分
（一）子查询推进（Push Subquery）示例
1LHR@orclasm > set serveroutput on
2LHR@orclasm > exec sql_explain('SELECT /*+ no_push_subq(@lhr_ps)*/ * FROM SYS.TAB$ A WHERE A.ANALYZETIME > (SELECT /*+qb_name(lhr_ps)*/ MAX(B.ANALYZETIME) FROM SYS.IND$ B)','outline');
3Plan hash value: 553156288
4----------------------------------------------------------------------------
5| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
6----------------------------------------------------------------------------
7| 0 | SELECT STATEMENT | | 3322 | 454K| 1118 (1)| 00:00:14 |
8|* 1 | FILTER | | | | | |
9| 2 | TABLE ACCESS FULL | TAB$ | 3322 | 454K| 559 (1)| 00:00:07 |
10| 3 | SORT AGGREGATE | | 1 | 7 | | |
11| 4 | TABLE ACCESS FULL| IND$ | 5545 | 38815 | 559 (1)| 00:00:07 |
12----------------------------------------------------------------------------
13Outline Data
14-------------
15/*+
16BEGIN_OUTLINE_DATA
17FULL(@"LHR_PS" "B"@"LHR_PS")
18FULL(@"SEL$1" "A"@"SEL$1")
19OUTLINE(@"LHR_PS")
20OUTLINE_LEAF(@"SEL$1")
21OUTLINE_LEAF(@"LHR_PS")
22ALL_ROWS
23DB_VERSION('11.2.0.3')
24OPTIMIZER_FEATURES_ENABLE('11.2.0.3')
25IGNORE_OPTIM_EMBEDDED_HINTS
26END_OUTLINE_DATA
27*/
28Predicate Information (identified by operation id):
29---------------------------------------------------
301 - filter("A"."ANALYZETIME"> (SELECT /*+ NO_PUSH_SUBQ QB_NAME
31("LHR_PS") */ MAX("B"."ANALYZETIME") FROM "SYS"."IND$" "B"))
32
33PL/SQL procedure successfully completed.
34
35LHR@orclasm > exec sql_explain('SELECT * FROM SYS.TAB$ A WHERE A.ANALYZETIME > (SELECT MAX(B.ANALYZETIME) FROM SYS.IND$ B)','outline');
36Plan hash value: 243387038
37----------------------------------------------------------------------------
38| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
39----------------------------------------------------------------------------
40| 0 | SELECT STATEMENT | | 166 | 23240 | 1118 (1)| 00:00:14 |
41|* 1 | TABLE ACCESS FULL | TAB$ | 166 | 23240 | 559 (1)| 00:00:07 |
42| 2 | SORT AGGREGATE | | 1 | 7 | | |
43| 3 | TABLE ACCESS FULL| IND$ | 5545 | 38815 | 559 (1)| 00:00:07 |
44----------------------------------------------------------------------------
45Outline Data
46-------------
47/*+
48BEGIN_OUTLINE_DATA
49FULL(@"SEL$2" "B"@"SEL$2")
50PUSH_SUBQ(@"SEL$2")
51FULL(@"SEL$1" "A"@"SEL$1")
52OUTLINE_LEAF(@"SEL$1")
53OUTLINE_LEAF(@"SEL$2")
54ALL_ROWS
55DB_VERSION('11.2.0.3')
56OPTIMIZER_FEATURES_ENABLE('11.2.0.3')
57IGNORE_OPTIM_EMBEDDED_HINTS
58END_OUTLINE_DATA
59*/
60Predicate Information (identified by operation id):
61---------------------------------------------------
621 - filter("A"."ANALYZETIME"> (SELECT MAX("B"."ANALYZETIME") FROM "SYS"."IND$" "B"))
63
64PL/SQL procedure successfully completed.
（二）子查询展开（Subquery Unnesting）
1--1）IN和EXISTS转换为半连接（SEMI JOIN）：
2CREATE TABLE EMP_LHR AS SELECT * FROM SCOTT.EMP;
3CREATE TABLE DEPT_LHR AS SELECT * FROM SCOTT.DEPT;
4SELECT * FROM EMP_LHR A WHERE EXISTS (SELECT 1 FROM DEPT_LHR B WHERE B.DEPTNO=A.DEPTNO);
5-------------------------------------------------------------------------------
6| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
7-------------------------------------------------------------------------------
8| 0 | SELECT STATEMENT | | 14 | 1400 | 7 (15)| 00:00:01 |
9|* 1 | HASH JOIN SEMI | | 14 | 1400 | 7 (15)| 00:00:01 |
10| 2 | TABLE ACCESS FULL| EMP_LHR | 14 | 1218 | 3 (0)| 00:00:01 |
11| 3 | TABLE ACCESS FULL| DEPT_LHR | 4 | 52 | 3 (0)| 00:00:01 |
12-------------------------------------------------------------------------------
13
14--子查询引用表DEPT，最终转换为两个表的哈希半连接。也就是说，EXISTS子句中的子查询被展开，其中的对象与主查询中的对象直接进行半关联操作。IN的情况类似，如下：
15SELECT * FROM EMP_LHR A WHERE A.DEPTNO IN (SELECT B.DEPTNO FROM DEPT_LHR B);
16-------------------------------------------------------------------------------
17| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
18-------------------------------------------------------------------------------
19| 0 | SELECT STATEMENT | | 14 | 1400 | 7 (15)| 00:00:01 |
20|* 1 | HASH JOIN SEMI | | 14 | 1400 | 7 (15)| 00:00:01 |
21| 2 | TABLE ACCESS FULL| EMP_LHR | 14 | 1218 | 3 (0)| 00:00:01 |
22| 3 | TABLE ACCESS FULL| DEPT_LHR | 4 | 52 | 3 (0)| 00:00:01 |
23-------------------------------------------------------------------------------
24
25--2）NOT IN和NOT EXISTS转换为反连接（ANTI JOIN）：
26SELECT * FROM EMP_LHR A WHERE NOT EXISTS (SELECT 1 FROM DEPT_LHR B WHERE B.DEPTNO=A.DEPTNO);
27-------------------------------------------------------------------------------
28| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
29-------------------------------------------------------------------------------
30| 0 | SELECT STATEMENT | | 14 | 1400 | 7 (15)| 00:00:01 |
31|* 1 | HASH JOIN ANTI | | 14 | 1400 | 7 (15)| 00:00:01 |
32| 2 | TABLE ACCESS FULL| EMP_LHR | 14 | 1218 | 3 (0)| 00:00:01 |
33| 3 | TABLE ACCESS FULL| DEPT_LHR | 4 | 52 | 3 (0)| 00:00:01 |
34-------------------------------------------------------------------------------
35
36--优化器将NOT EXISTS后的子查询做解嵌套，然后选择了哈希的反连接。这种转换属于基于代价的查询转换。下面看看NOT IN的情况：
37SELECT * FROM EMP_LHR A WHERE A.DEPTNO NOT IN (SELECT B.DEPTNO FROM DEPT_LHR B);
38-------------------------------------------------------------------------------
39| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
40-------------------------------------------------------------------------------
41| 0 | SELECT STATEMENT | | 14 | 1400 | 7 (15)| 00:00:01 |
42|* 1 | HASH JOIN ANTI NA | | 14 | 1400 | 7 (15)| 00:00:01 |
43| 2 | TABLE ACCESS FULL| EMP_LHR | 14 | 1218 | 3 (0)| 00:00:01 |
44| 3 | TABLE ACCESS FULL| DEPT_LHR | 4 | 52 | 3 (0)| 00:00:01 |
45-------------------------------------------------------------------------------
46
和NOT EXISTS类似，也选择了哈希连接，只不过是HASH JOIN ANTI NA。这里的NA，实际表示Null-Aware的意思，在11g及以后的版本中，Oracle增加了对空值敏感的反关联的支持。
（三）子查询合并（Subquery Coalesce）
1LHR@orclasm > SELECT * FROM EMP_LHR A WHERE EXISTS (SELECT 1 FROM DEPT_LHR B WHERE B.DEPTNO=A.DEPTNO AND B.DEPTNO=10) AND EXISTS (SELECT 1 FROM DEPT_LHR B WHERE B.DEPTNO=A.DEPTNO AND B.DEPTNO=20) ;
2
3no rows selected
4
5
6Execution Plan
7----------------------------------------------------------
8Plan hash value: 3115025369
9
10---------------------------------------------------------------------------------
11| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
12---------------------------------------------------------------------------------
13| 0 | SELECT STATEMENT | | 1 | 113 | 0 (0)| |
14|* 1 | FILTER | | | | | |
15|* 2 | HASH JOIN SEMI | | 1 | 113 | 10 (10)| 00:00:01 |
16|* 3 | HASH JOIN SEMI | | 1 | 100 | 7 (15)| 00:00:01 |
17|* 4 | TABLE ACCESS FULL| EMP_LHR | 1 | 87 | 3 (0)| 00:00:01 |
18|* 5 | TABLE ACCESS FULL| DEPT_LHR | 1 | 13 | 3 (0)| 00:00:01 |
19|* 6 | TABLE ACCESS FULL | DEPT_LHR | 1 | 13 | 3 (0)| 00:00:01 |
20---------------------------------------------------------------------------------
21
22Predicate Information (identified by operation id):
23---------------------------------------------------
24
25 1 - filter(NULL IS NOT NULL)
26 2 - access("B"."DEPTNO"="A"."DEPTNO")
27 3 - access("B"."DEPTNO"="A"."DEPTNO")
28 4 - filter("A"."DEPTNO"=10 AND "A"."DEPTNO"=20)
29 5 - filter("B"."DEPTNO"=20 AND "B"."DEPTNO"=10)
30 6 - filter("B"."DEPTNO"=10 AND "B"."DEPTNO"=20)
在这个查询语句中，外部查询要满足两个子查询—SUB1和SUB2，但两者条件不同，不能简单合并。因此在执行计划中，分别对两者进行了扫描（直观感觉就是对DEPT_LHR进行了两次扫描），然后再做关联查询。
1LHR@orclasm > SELECT * FROM EMP_LHR A WHERE EXISTS (SELECT 1 FROM DEPT_LHR B WHERE B.DEPTNO=A.DEPTNO AND B.DEPTNO=10) AND EXISTS (SELECT 1 FROM DEPT_LHR B WHERE B.DEPTNO=A.DEPTNO) ;
2
3 EMPNO ENAME JOB MGR HIREDATE SAL COMM DEPTNO
4---------- ---------- --------- ---------- ------------------- ---------- ---------- ----------
5 7934 MILLER CLERK 7782 1982-01-23 00:00:00 1300 10
6 7839 KING PRESIDENT 1981-11-17 00:00:00 5000 10
7 7782 CLARK MANAGER 7839 1981-06-09 00:00:00 2450 10
8
9
10Execution Plan
11----------------------------------------------------------
12Plan hash value: 3403691855
13
14-------------------------------------------------------------------------------
15| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
16-------------------------------------------------------------------------------
17| 0 | SELECT STATEMENT | | 1 | 100 | 7 (15)| 00:00:01 |
18|* 1 | HASH JOIN SEMI | | 1 | 100 | 7 (15)| 00:00:01 |
19|* 2 | TABLE ACCESS FULL| EMP_LHR | 3 | 261 | 3 (0)| 00:00:01 |
20|* 3 | TABLE ACCESS FULL| DEPT_LHR | 1 | 13 | 3 (0)| 00:00:01 |
21-------------------------------------------------------------------------------
22
23Predicate Information (identified by operation id):
24---------------------------------------------------
25
26 1 - access("B"."DEPTNO"="A"."DEPTNO")
27 2 - filter("A"."DEPTNO"=10)
28 3 - filter("B"."DEPTNO"=10)
在这个查询中，外部对EMP_LHR表的查询要同时满足SUB1和SUB2两个子查询，而SUB1在语义上又是SUB2的子集，因此优化器将两个子查询进行了合并（只进行一次对DEPT_LHR表的扫描），然后与外部表EMP_LHR进行半连接。
本文选自《Oracle程序员面试笔试宝典》，作者：李华荣。
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