【DB笔试面试616】在Oracle中，和“消除”相关的查询转换有哪些？
2019-08-11 10:00
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（二）去重消除
（三）表消除
（四）公共子表达式消除（Common Sub-expression Elimination，CSE）
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在Oracle中，和“消除”相关的查询转换有哪些？
[strong]♣[/strong]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: 966064101112-----------------------------------------------------------------------------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-----------------------------------------------------------------------------192021Statistics22----------------------------------------------------------23 47 recursive calls24 5 db block gets25 1 consistent gets26 0 physical reads27 2616 redo size28 526 bytes sent via SQL*Net to client29 519 bytes received via SQL*Net from client30 2 SQL*Net roundtrips to/from client31 0 sorts (memory)32 0 sorts (disk)33 1 rows processedSELECT 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----------------------------------------------------------------------------------------1617Note18-----19 - dynamic sampling used for this statement (level=2)202122Statistics23----------------------------------------------------------24 6 recursive calls25 0 db block gets26 9 consistent gets27 1 physical reads28 0 redo size29 1710 bytes sent via SQL*Net to client30 552 bytes received via SQL*Net from client31 5 SQL*Net roundtrips to/from client32 0 sorts (memory)33 0 sorts (disk)34 58 rows processed3536ALTER TABLE T_QC_20170613_LHR ADD PRIMARY KEY (USERNAME);3738LHR@orclasm > SELECT DISTINCT T.USERNAME FROM T_QC_20170613_LHR T; 3940Execution Plan41----------------------------------------------------------42Plan hash value: 8848138324344---------------------------------------------------------------------------------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---------------------------------------------------------------------------------5051Note52-----53 - dynamic sampling used for this statement (level=2)545556Statistics57----------------------------------------------------------58 79 recursive calls59 28 db block gets60 83 consistent gets61 0 physical reads62 0 redo size63 1710 bytes sent via SQL*Net to client64 552 bytes received via SQL*Net from client65 5 SQL*Net roundtrips to/from client66 6 sorts (memory)67 0 sorts (disk)68 58 rows processedCREATE 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---------------------------------------------------1112 1 - filter("A"."DEPTNO" IS NOT NULL)SELECT 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---------------------------------------------------------------------------1112Predicate Information (identified by operation id):13---------------------------------------------------1415 1 - access("A"."DEPTNO"="B"."DEPTNO")16 3 - filter("A"."EMPNO"<=7521 OR "A"."EMPNO">=7782)1718LHR@orclasm > ALTER SESSION SET "_ELIMINATE_COMMON_SUBEXPR"=FALSE; 1920Session altered.2122LHR@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 );2324---------------------------------------------------------------------------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---------------------------------------------------------------------------3233Predicate Information (identified by operation id):34---------------------------------------------------3536 3 - filter("A"."DEPTNO"="B"."DEPTNO" AND "A"."EMPNO"<=7521 OR37 "A"."DEPTNO"="B"."DEPTNO" AND "A"."EMPNO">=7782)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)
本文选自《Oracle程序员面试笔试宝典》，作者：李华荣。
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