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昨天在书上看到SQL语句优化时，where条件顺序不同，性能不同，这个建议在Oracle11G版本还合适吗？方式1优于方式2？ 方式1： 复制代码 代码如下: select a.* from students s, class c where s.id = c.id s.id = 'xxxxxxxx' 方式2： 复制代码 代码如下: select a.* from students s, class c where s.id = 'xxxxxxxx' s.id = c.id 10g中测试结果证明是一样的。 Microsoft Windows [版本 5.2.3790] (C) 版权所有 1985-2003 Microsoft Corp. C:\Documents and Settings\Administrator>sqlplus / as sysdba SQL*Plus: Release 10.2.0.1.0 - Production on 星期六 5月 11 17:48:55 2013 Copyright (c) 1982, 2005, Oracle. All rights reserved. 连接到: Oracle Database 10g Enterprise Edition Release 10.2.0.1.0 - Production With the Partitioning, OLAP and Data Mining options SQL> alter system flush shared_pool; 系统已更改。 SQL> alter system flush buffer_cache; 系统已更改。 SQL> set autotrace on; SQL> select * 2 from COUNTRIES c, 3 REGIONS r 4 where c.REGION_ID=r.REGION_ID and c.REGION_ID='4'; REGIONS r * 第 3 行出现错误: ORA-00942: 表或视图不存在 SQL> select * 2 from hr.COUNTRIES c, 3 hr. REGIONS r 4 where c.REGION_ID=r.REGION_ID and c.REGION_ID='4'; CO COUNTRY_NAME REGION_ID REGION_ID -- ---------------------------------------- ---------- ---------- REGION_NAME ------------------------- EG Egypt 4 4 Middle East and Africa IL Israel 4 4 Middle East and Africa KW Kuwait 4 4 Middle East and Africa CO COUNTRY_NAME REGION_ID REGION_ID -- ---------------------------------------- ---------- ---------- REGION_NAME ------------------------- NG Nigeria 4 4 Middle East and Africa ZM Zambia 4 4 Middle East and Africa ZW Zimbabwe 4 4 Middle East and Africa 已选择6行。 执行计划 ---------------------------------------------------------- Plan hash value: 4030513296 -------------------------------------------------------------------------------- ---------------- | Id | Operation | Name | Rows | Bytes | Cost (% CPU)| Time | -------------------------------------------------------------------------------- ---------------- | 0 | SELECT STATEMENT | | 6 | 168 | 2 (0)| 00:00:01 | | 1 | NESTED LOOPS | | 6 | 168 | 2 (0)| 00:00:01 | | 2 | TABLE ACCESS BY INDEX ROWID| REGIONS | 1 | 14 | 1 (0)| 00:00:01 | |* 3 | INDEX UNIQUE SCAN | REG_ID_PK | 1 | | 0 (0)| 00:00:01 | |* 4 | INDEX FULL SCAN | COUNTRY_C_ID_PK | 6 | 84 | 1 (0)| 00:00:01 | -------------------------------------------------------------------------------- ---------------- Predicate Information (identified by operation id): --------------------------------------------------- 3 - access("R"."REGION_ID"=4) 4 - filter("C"."REGION_ID"=4) 统计信息 ---------------------------------------------------------- 628 recursive calls 0 db block gets 127 consistent gets 20 physical reads 0 redo size 825 bytes sent via SQL*Net to client 385 bytes received via SQL*Net from client 2 SQL*Net roundtrips to/from client 13 sorts (memory) 0 sorts (disk) 6 rows processed SQL> ############# SQL> alter system flush shared_pool; 系统已更改。 SQL> alter system flush buffer_cache; 系统已更改。 select * from hr.COUNTRIES c, hr. REGIONS r where c.REGION_ID='4' 6 and c.REGION_ID=r.REGION_ID; CO COUNTRY_NAME REGION_ID REGION_ID -- ---------------------------------------- ---------- ---------- REGION_NAME ------------------------- EG Egypt 4 4 Middle East and Africa IL Israel 4 4 Middle East and Africa KW Kuwait 4 4 Middle East and Africa CO COUNTRY_NAME REGION_ID REGION_ID -- ---------------------------------------- ---------- ---------- REGION_NAME ------------------------- NG Nigeria 4 4 Middle East and Africa ZM Zambia 4 4 Middle East and Africa ZW Zimbabwe 4 4 Middle East and Africa 已选择6行。 执行计划 ---------------------------------------------------------- Plan hash value: 4030513296 -------------------------------------------------------------------------------- ---------------- | Id | Operation | Name | Rows | Bytes | Cost (% CPU)| Time | -------------------------------------------------------------------------------- ---------------- | 0 | SELECT STATEMENT | | 6 | 168 | 2 (0)| 00:00:01 | | 1 | NESTED LOOPS | | 6 | 168 | 2 (0)| 00:00:01 | | 2 | TABLE ACCESS BY INDEX ROWID| REGIONS | 1 | 14 | 1 (0)| 00:00:01 | |* 3 | INDEX UNIQUE SCAN | REG_ID_PK | 1 | | 0 (0)| 00:00:01 | |* 4 | INDEX FULL SCAN | COUNTRY_C_ID_PK | 6 | 84 | 1 (0)| 00:00:01 | -------------------------------------------------------------------------------- ---------------- Predicate Information (identified by operation id): --------------------------------------------------- 3 - access("R"."REGION_ID"=4) 4 - filter("C"."REGION_ID"=4) 统计信息 ---------------------------------------------------------- 656 recursive calls 0 db block gets 131 consistent gets 22 physical reads 0 redo size 825 bytes sent via SQL*Net to client 385 bytes received via SQL*Net from client 2 SQL*Net roundtrips to/from client 13 sorts (memory) 0 sorts (disk) 6 rows processed SQL> |
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