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 calls 0 db block gets 2490 consistent gets 2484 physical reads 0 redo size 528 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) 1 rows processed SQL> 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 calls 0 db block gets 2629 consistent gets 2484 physical reads 0 redo size 528 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) 1 rows processed SQL> 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 calls 0 db block gets 4691 consistent gets 3726 physical reads 0 redo size 528 bytes sent via SQL*Net to client 524 bytes received via SQL*Net from client 2 SQL*Net roundtrips to/from client 12 sorts (memory) 0 sorts (disk) 1 rows processed SQL> 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 calls 0 db block gets 4669 consistent gets 3726 physical reads 0 redo size 527 bytes sent via SQL*Net to client 524 bytes received via SQL*Net from client 2 SQL*Net roundtrips to/from client 8 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 calls 0 db block gets 1315 consistent gets 1242 physical reads 0 redo size 526 bytes sent via SQL*Net to client 524 bytes received via SQL*Net from client 2 SQL*Net roundtrips to/from client 1 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 CONNECT BY LEVEL<100)) 4 - filter(LEVEL=:B1) 5 - filter(LEVEL<100) Note----- - dynamic sampling used for this statement (level=2) Statistics---------------------------------------------------------- 7 recursive calls 0 db block gets 1385 consistent gets 1242 physical reads 0 redo size 526 bytes sent via SQL*Net to client 524 bytes received via SQL*Net from client 2 SQL*Net roundtrips to/from client 86968 sorts (memory) 0 sorts (disk) 1 rows processed
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