作者：Jerry

有时候优化sql的时候，在执行计划中看到有VM_NSO_X的视图，在Oracle定义中，可以吧NSO理解为nested subquery optimizing，功能就是把in转换为join，把not in转换为anti join等，当然转换的时候有一定的限制。下面我们来简单看下会生成VM_NSO_1视图的几个例子

1. 创建2个测试表

SQL> create table test_jerry as select * from dba_objects;Table created.SQL> create table test_jerry2 as select * from dba_objects;Table created.SQL> select count(*) from test_jerry where object_id not in (select max(object_id) from test_jerry2 group by owner);Execution Plan
----------------------------------------------------------
Plan hash value: 3525080607------------------------------------------------------------------------------------------------
| Id  | Operation                | Name        | Rows  | Bytes |TempSpc| Cost (%CPU)| Time     |
------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT         |             |     1 |    26 |       |   908   (1)| 00:00:11 |
|   1 |  SORT AGGREGATE          |             |     1 |    26 |       |            |          |
|*  2 |   HASH JOIN RIGHT ANTI NA|             | 89846 |  2281K|  2144K|   908   (1)| 00:00:11 |
|   3 |    VIEW                  | VW_NSO_1    | 87509 |  1110K|       |   349   (1)| 00:00:05 |
|   4 |     HASH GROUP BY        |             | 87509 |  2563K|       |   349   (1)| 00:00:05 |
|   5 |      TABLE ACCESS FULL   | TEST_JERRY2 | 87509 |  2563K|       |   347   (1)| 00:00:05 |
|   6 |    TABLE ACCESS FULL     | TEST_JERRY  | 89847 |  1140K|       |   347   (1)| 00:00:05 |
------------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------2 - access("OBJECT_ID"="MAX(OBJECT_ID)")Note
------ dynamic sampling used for this statement (level=2)Statistics
----------------------------------------------------------0  recursive calls0  db block gets2490  consistent gets2484  physical reads0  redo size528  bytes sent via SQL*Net to client524  bytes received via SQL*Net from client2  SQL*Net roundtrips to/from client0  sorts (memory)0  sorts (disk)1  rows processedSQL> select count(*) from test_jerry where object_id not in (select max(object_id) from test_jerry2);Execution Plan
----------------------------------------------------------
Plan hash value: 3071647562------------------------------------------------------------------------------------
| Id  | Operation            | Name        | Rows  | Bytes | Cost (%CPU)| Time     |
------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT     |             |     1 |    13 |   694   (1)| 00:00:09 |
|   1 |  SORT AGGREGATE      |             |     1 |    13 |            |          |
|*  2 |   TABLE ACCESS FULL  | TEST_JERRY  |  4492 | 58396 |   347   (1)| 00:00:05 |
|   3 |    SORT AGGREGATE    |             |     1 |    13 |            |          |
|   4 |     TABLE ACCESS FULL| TEST_JERRY2 | 87509 |  1110K|   347   (1)| 00:00:05 |
------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------2 - filter("OBJECT_ID"<> (SELECT MAX("OBJECT_ID") FROM "TEST_JERRY2""TEST_JERRY2"))Note
------ dynamic sampling used for this statement (level=2)Statistics
----------------------------------------------------------7  recursive calls0  db block gets2629  consistent gets2484  physical reads0  redo size528  bytes sent via SQL*Net to client524  bytes received via SQL*Net from client2  SQL*Net roundtrips to/from client0  sorts (memory)0  sorts (disk)1  rows processedSQL> select count(*) from test_jerry where object_id in (select object_id from test_jerry2 where owner='SYS' minus select object_id from test_jerry where owner='SCOTT');Execution Plan
----------------------------------------------------------
Plan hash value: 773093838----------------------------------------------------------------------------------------------
| Id  | Operation              | Name        | Rows  | Bytes |TempSpc| Cost (%CPU)| Time     |
----------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT       |             |     1 |    26 |       |  1307   (1)| 00:00:16 |
|   1 |  SORT AGGREGATE        |             |     1 |    26 |       |            |          |
|*  2 |   HASH JOIN            |             | 32153 |   816K|       |  1307   (1)| 00:00:16 |
|   3 |    VIEW                | VW_NSO_1    | 32153 |   408K|       |   960   (1)| 00:00:12 |
|   4 |     MINUS              |             |       |       |       |            |          |
|   5 |      SORT UNIQUE       |             | 32153 |   941K|  1272K|            |          |
|*  6 |       TABLE ACCESS FULL| TEST_JERRY2 | 32153 |   941K|       |   347   (1)| 00:00:05 |
|   7 |      SORT UNIQUE       |             |    14 |   420 |       |            |          |
|*  8 |       TABLE ACCESS FULL| TEST_JERRY  |    14 |   420 |       |   347   (1)| 00:00:05 |
|   9 |    TABLE ACCESS FULL   | TEST_JERRY  | 89847 |  1140K|       |   347   (1)| 00:00:05 |
----------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------2 - access("OBJECT_ID"="OBJECT_ID")6 - filter("OWNER"='SYS')8 - filter("OWNER"='SCOTT')Note
------ dynamic sampling used for this statement (level=2)Statistics
----------------------------------------------------------93  recursive calls0  db block gets4691  consistent gets3726  physical reads0  redo size528  bytes sent via SQL*Net to client524  bytes received via SQL*Net from client2  SQL*Net roundtrips to/from client12  sorts (memory)0  sorts (disk)1  rows processedSQL> select count(*) from test_jerry where object_id in (select object_id from test_jerry2 where owner='SYS' union all select object_id from test_jerry where owner='SCOTT');Execution Plan
----------------------------------------------------------
Plan hash value: 1173723582--------------------------------------------------------------------------------------
| Id  | Operation              | Name        | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT       |             |     1 |    26 |  1041   (1)| 00:00:13 |
|   1 |  SORT AGGREGATE        |             |     1 |    26 |            |          |
|*  2 |   HASH JOIN            |             | 32167 |   816K|  1041   (1)| 00:00:13 |
|   3 |    VIEW                | VW_NSO_1    | 32167 |   408K|   694   (1)| 00:00:09 |
|   4 |     HASH UNIQUE        |             | 32167 |   942K|   694   (1)| 00:00:09 |
|   5 |      UNION-ALL         |             |       |       |            |          |
|*  6 |       TABLE ACCESS FULL| TEST_JERRY2 | 32153 |   941K|   347   (1)| 00:00:05 |
|*  7 |       TABLE ACCESS FULL| TEST_JERRY  |    14 |   420 |   347   (1)| 00:00:05 |
|   8 |    TABLE ACCESS FULL   | TEST_JERRY  | 89847 |  1140K|   347   (1)| 00:00:05 |
--------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------2 - access("OBJECT_ID"="OBJECT_ID")6 - filter("OWNER"='SYS')7 - filter("OWNER"='SCOTT')Note
------ dynamic sampling used for this statement (level=2)Statistics
----------------------------------------------------------82  recursive calls0  db block gets4669  consistent gets3726  physical reads0  redo size527  bytes sent via SQL*Net to client524  bytes received via SQL*Net from client2  SQL*Net roundtrips to/from client8  sorts (memory)0  sorts (disk)1  rows processed
SQL> select count(*) from test_jerry where object_id in (SELECT LEVEL FROM DUAL CONNECT BY LEVEL<100);  Execution Plan
----------------------------------------------------------
Plan hash value: 3708743834--------------------------------------------------------------------------------------------------------
| Id  | Operation                                 | Name       | Rows  | Bytes | Cost (%CPU)| Time     |
--------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT                          |            |     1 |    26 |   350   (1)| 00:00:05 |
|   1 |  SORT AGGREGATE                           |            |     1 |    26 |            |          |
|*  2 |   HASH JOIN                               |            |     1 |    26 |   350   (1)| 00:00:05 |
|   3 |    VIEW                                   | VW_NSO_1   |     1 |    13 |     3  (34)| 00:00:01 |
|   4 |     HASH UNIQUE                           |            |     1 |       |     3  (34)| 00:00:01 |
|*  5 |      CONNECT BY WITHOUT FILTERING (UNIQUE)|            |       |       |            |          |
|   6 |       FAST DUAL                           |            |     1 |       |     2   (0)| 00:00:01 |
|   7 |    TABLE ACCESS FULL                      | TEST_JERRY | 89847 |  1140K|   347   (1)| 00:00:05 |
--------------------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------2 - access("OBJECT_ID"="LEVEL")5 - filter(LEVEL<100)Note
------ dynamic sampling used for this statement (level=2)Statistics
----------------------------------------------------------4  recursive calls0  db block gets1315  consistent gets1242  physical reads0  redo size526  bytes sent via SQL*Net to client524  bytes received via SQL*Net from client2  SQL*Net roundtrips to/from client1  sorts (memory)0  sorts (disk)1  rows processed

可以从上面的sql得出一个简单的结论，当子查询中出现max，rownum，group by，union all，minus，intersect等聚合函数的时候，Oracle就会自动把子查询转换成视图VM_NSO_X，其实在Oracle的子查询中如果出现上面的几种情况，也就限制了view merge，就无法对视图进行merge。如果需要去掉CBO生成VM_NSO_1，只需要在子查询中加一个no_unnest限制，这样CBO就会走filter了。下面看一个加了no_unnest的执行计划

SQL> select count(*) from test_jerry where object_id in (SELECT /*+ no_unnest() */ LEVEL FROM DUAL t CONNECT BY LEVEL<100);Execution Plan
----------------------------------------------------------
Plan hash value: 2000702637-------------------------------------------------------------------------------------------------------
| Id  | Operation                                | Name       | Rows  | Bytes | Cost (%CPU)| Time     |
-------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT                         |            |     1 |    13 |  5963   (1)| 00:01:12 |
|   1 |  SORT AGGREGATE                          |            |     1 |    13 |            |          |
|*  2 |   FILTER                                 |            |       |       |            |          |
|   3 |    TABLE ACCESS FULL                     | TEST_JERRY | 89847 |  1140K|   347   (1)| 00:00:05 |
|*  4 |    FILTER                                |            |       |       |            |          |
|*  5 |     CONNECT BY WITHOUT FILTERING (UNIQUE)|            |       |       |            |          |
|   6 |      FAST DUAL                           |            |     1 |       |     2   (0)| 00:00:01 |
-------------------------------------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------2 - filter( EXISTS (SELECT /*+ NO_UNNEST */ 0 FROM "SYS"."DUAL" "T" WHERE LEVEL=:B1 CONNECTBY LEVEL<100))4 - filter(LEVEL=:B1)5 - filter(LEVEL<100)Note
------ dynamic sampling used for this statement (level=2)Statistics
----------------------------------------------------------7  recursive calls0  db block gets1385  consistent gets1242  physical reads0  redo size526  bytes sent via SQL*Net to client524  bytes received via SQL*Net from client2  SQL*Net roundtrips to/from client86968  sorts (memory)0  sorts (disk)1  rows processed

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转载于:https://www.cnblogs.com/lxl57610/p/7464117.html

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