在Oracle中，和“消除”相关的查询转换有哪些？
♣ 答案部分
（一）排序消除
1LHR@orclasm > SELECT COUNT(1) FROM ( SELECT T.EMPNO FROM SCOTT.EMP T ORDER BY T.EMPNO); 2 3 COUNT(1) 4---------- 5 14 6 7 8Execution Plan 9---------------------------------------------------------- 10Plan hash value: 96606410 11 12----------------------------------------------------------------------------- 13| Id | Operation | Name | Rows | Cost (%CPU)| Time | 14----------------------------------------------------------------------------- 15| 0 | SELECT STATEMENT | | 1 | 1 (0)| 00:00:01 | 16| 1 | SORT AGGREGATE | | 1 | | | 17| 2 | INDEX FULL SCAN| IDX_FULL_EMP_LHR | 14 | 1 (0)| 00:00:01 | 18----------------------------------------------------------------------------- 19 20 21Statistics 22---------------------------------------------------------- 23 47 recursive calls 24 5 db block gets 25 1 consistent gets 26 0 physical reads 27 2616 redo size 28 526 bytes sent via SQL*Net to client 29 519 bytes received via SQL*Net from client 30 2 SQL*Net roundtrips to/from client 31 0 sorts (memory) 32 0 sorts (disk) 33 1 rows processed
（二）去重消除
1CREATE TABLE T_QC_20170613_LHR AS SELECT * FROM DBA_USERS; 2 3LHR@orclasm > SELECT DISTINCT T.USERNAME FROM T_QC_20170613_LHR T; 4 5Execution Plan 6---------------------------------------------------------- 7Plan hash value: 1708573004 8 9---------------------------------------------------------------------------------------- 10| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | 11---------------------------------------------------------------------------------------- 12| 0 | SELECT STATEMENT | | 58 | 986 | 4 (25)| 00:00:01 | 13| 1 | HASH UNIQUE | | 58 | 986 | 4 (25)| 00:00:01 | 14| 2 | TABLE ACCESS FULL| T_QC_20170613_LHR | 58 | 986 | 3 (0)| 00:00:01 | 15---------------------------------------------------------------------------------------- 16 17Note 18----- 19 - dynamic sampling used for this statement (level=2) 20 21 22Statistics 23---------------------------------------------------------- 24 6 recursive calls 25 0 db block gets 26 9 consistent gets 27 1 physical reads 28 0 redo size 29 1710 bytes sent via SQL*Net to client 30 552 bytes received via SQL*Net from client 31 5 SQL*Net roundtrips to/from client 32 0 sorts (memory) 33 0 sorts (disk) 34 58 rows processed 35 36ALTER TABLE T_QC_20170613_LHR ADD PRIMARY KEY (USERNAME); 37 38LHR@orclasm > SELECT DISTINCT T.USERNAME FROM T_QC_20170613_LHR T; 39 40Execution Plan 41---------------------------------------------------------- 42Plan hash value: 884813832 43 44--------------------------------------------------------------------------------- 45| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | 46--------------------------------------------------------------------------------- 47| 0 | SELECT STATEMENT | | 58 | 986 | 1 (0)| 00:00:01 | 48| 1 | INDEX FULL SCAN | SYS_C0089569 | 58 | 986 | 1 (0)| 00:00:01 | 49--------------------------------------------------------------------------------- 50 51Note 52----- 53 - dynamic sampling used for this statement (level=2) 54 55 56Statistics 57---------------------------------------------------------- 58 79 recursive calls 59 28 db block gets 60 83 consistent gets 61 0 physical reads 62 0 redo size 63 1710 bytes sent via SQL*Net to client 64 552 bytes received via SQL*Net from client 65 5 SQL*Net roundtrips to/from client 66 6 sorts (memory) 67 0 sorts (disk) 68 58 rows processed
（三）表消除
1SELECT A.* FROM SCOTT.EMP A,SCOTT.DEPT B WHERE A.DEPTNO=B.DEPTNO;
2--------------------------------------------------------------------------
3| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
4--------------------------------------------------------------------------
5| 0 | SELECT STATEMENT | | 14 | 1218 | 3 (0)| 00:00:01 |
6|* 1 | TABLE ACCESS FULL| EMP | 14 | 1218 | 3 (0)| 00:00:01 |
7--------------------------------------------------------------------------
8
9Predicate Information (identified by operation id):
10---------------------------------------------------
11
12 1 - filter("A"."DEPTNO" IS NOT NULL)
（四）公共子表达式消除（Common Sub-expression Elimination，CSE）
1LHR@orclasm > SELECT * FROM SCOTT.EMP A,SCOTT.DEPT B WHERE ( A.DEPTNO=B.DEPTNO AND A.EMPNO<=7521) OR ( A.DEPTNO=B.DEPTNO AND A.EMPNO>=7782 );
2
3---------------------------------------------------------------------------
4| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
5---------------------------------------------------------------------------
6| 0 | SELECT STATEMENT | | 11 | 1287 | 7 (15)| 00:00:01 |
7|* 1 | HASH JOIN | | 11 | 1287 | 7 (15)| 00:00:01 |
8| 2 | TABLE ACCESS FULL| DEPT | 4 | 120 | 3 (0)| 00:00:01 |
9|* 3 | TABLE ACCESS FULL| EMP | 11 | 957 | 3 (0)| 00:00:01 |
10---------------------------------------------------------------------------
11
12Predicate Information (identified by operation id):
13---------------------------------------------------
14
15 1 - access("A"."DEPTNO"="B"."DEPTNO")
16 3 - filter("A"."EMPNO"<=7521 OR "A"."EMPNO">=7782)
17
18LHR@orclasm > ALTER SESSION SET "_ELIMINATE_COMMON_SUBEXPR"=FALSE;
19
20Session altered.
21
22LHR@orclasm > SELECT * FROM SCOTT.EMP A,SCOTT.DEPT B WHERE ( A.DEPTNO=B.DEPTNO AND A.EMPNO<=7521) OR ( A.DEPTNO=B.DEPTNO AND A.EMPNO>=7782 );
23
24---------------------------------------------------------------------------
25| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
26---------------------------------------------------------------------------
27| 0 | SELECT STATEMENT | | 1 | 117 | 10 (0)| 00:00:01 |
28| 1 | NESTED LOOPS | | 1 | 117 | 10 (0)| 00:00:01 |
29| 2 | TABLE ACCESS FULL| DEPT | 4 | 120 | 3 (0)| 00:00:01 |
30|* 3 | TABLE ACCESS FULL| EMP | 1 | 87 | 2 (0)| 00:00:01 |
31---------------------------------------------------------------------------
32
33Predicate Information (identified by operation id):
34---------------------------------------------------
35
36 3 - filter("A"."DEPTNO"="B"."DEPTNO" AND "A"."EMPNO"<=7521 OR
37 "A"."DEPTNO"="B"."DEPTNO" AND "A"."EMPNO">=7782) 
