Oracle11G多列收集统计信息实验
在上一篇文章中解决了Oracle11g 多列收集统计信息--直方图的问题,现在来做一个实验验证,这是Oracle 11g的新特性。 SQL> select * from v$version; SQL> drop table test purge; SQL> create table test as select * from dba_objects;SQL> create index ind_tt_oo on test(owner,object_type); SQL> exec dbms_stats.gather_table_stats(user,'test',cascade => true); SQL> select owner,object_type,count(1) from test where owner = 'SYS' and object_type = 'CLUSTER' group by owner,object_type; OWNER OBJECT_TYPE COUNT(1) ------- -------------- ---------- SYS CLUSTER 10 --这种组合数据有10条 SQL> set autotrace traceonlySQL> select * from test where owner='SYS' and object_type='CLUSTER'; 执行计划 ---------------------------------------------------------- Plan hash value: 3811756094 ----------------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | ----------------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 298 | 28906 | 15 (0)| 00:00:01 | | 1 | TABLE ACCESS BY INDEX ROWID| TEST | 298 | 28906 | 15 (0)| 00:00:01 | |* 2 | INDEX RANGE SCAN | IND_TT_OO | 298 | | 2 (0)| 00:00:01 |--而评估出来是298条 ----------------------------------------------------------------------------------------- Predicate Information (identified by operation id): --------------------------------------------------- 2 - access("OWNER"='SYS' AND "OBJECT_TYPE"='CLUSTER') 统计信息 ---------------------------------------------------------- 1 recursive calls 0 db block gets 9 consistent gets 0 physical reads 0 redo size 2450 bytes sent via SQL*Net to client 519 bytes received via SQL*Net from client 2 SQL*Net roundtrips to/from client 0 sorts (memory) 0 sorts (disk) 10 rows processed SQL> set autotrace off SQL> declare v_name varchar2(30); begin v_name := dbms_stats.create_extended_stats(user,'(owner,object_type)'); end; / SQL> set autotrace traceonly SQL> exec dbms_stats.gather_table_stats(user,cascade => true,method_opt => 'FOR ALL COLUMNS SIZE SKEWONLY FOR COLUMNS (owner,object_type)',no_invalidate=>FALSE); SQL> select * from test where owner='SYS' and object_type='CLUSTER'; 执行计划 ---------------------------------------------------------- Plan hash value: 3811756094 ----------------------------------------------------------------------------------------- | Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time | ----------------------------------------------------------------------------------------- | 0 | SELECT STATEMENT | | 13 | 1417 | 2 (0)| 00:00:01 | | 1 | TABLE ACCESS BY INDEX ROWID| TEST | 13 | 1417 | 2 (0)| 00:00:01 | |* 2 | INDEX RANGE SCAN | IND_TT_OO | 13 | | 1 (0)| 00:00:01 |--评估出来是13条,基本接近 ----------------------------------------------------------------------------------------- Predicate Information (identified by operation id): --------------------------------------------------- 2 - access("OWNER"='SYS' AND "OBJECT_TYPE"='CLUSTER') 统计信息 ---------------------------------------------------------- 8 recursive calls 0 db block gets 11 consistent gets 0 physical reads 0 redo size 2450 bytes sent via SQL*Net to client 519 bytes received via SQL*Net from client 2 SQL*Net roundtrips to/from client 0 sorts (memory) 0 sorts (disk) 10 rows processed 总结:在Oracle优化SQL,很多时候是要让CBO评估准过滤数据的条数。 (编辑:李大同) 【声明】本站内容均来自网络,其相关言论仅代表作者个人观点,不代表本站立场。若无意侵犯到您的权利,请及时与联系站长删除相关内容! |