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索引与Null值对于Hints及执行计划的影响发表日期：2008-2-9
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由于B*Tree索引不存储Null值,所以在索引字段答应为空的情况下,某些Oracle查询不会使用索引.
很多时候,我们看似可以使用全索引扫描(Full Index Scan)的情况,可能Oracle就会因为Null值的存在而放弃索引.
在此情况下即使使用Hints,Oracle也不会使用索引,其根本原因就是因为Null值的存在.
我们看以下测试.
在username字段为Not Null时,Index Hints可以生效.
SQL> create table t as select username,passWord from dba_users;Table created.SQL> desc t
Name Null? Type ----------------------------------------- -------- ---------------------------- USERNAME NOT NULL VARCHAR2(30) PASSWORD VARCHAR2(30)SQL> create index i_t on t(username);Index created.SQL> set autotrace trace eXPlain SQL> select * from t where username='EYGLE';Execution Plan ---------------------------------------------------------- Plan hash value: 1601196873-------------------------------------------------------------------------- Id Operation Name Rows Bytes Cost (%CPU) Time -------------------------------------------------------------------------- 0 SELECT STATEMENT 1 34 2 (0) 00:00:01 * 1 TABLE Access FULL T 1 34 2 (0) 00:00:01 --------------------------------------------------------------------------Predicate Information (identified by operation id): --------------------------------------------------- 1 - filter("USERNAME"='EYGLE')Note ----- - dynamic sampling used for this statementSQL> set linesize 120 SQL> select /*+ index(t,i_t) */ * from t where username='EYGLE';Execution Plan ---------------------------------------------------------- Plan hash value: 2928007915------------------------------------------------------------------------------------ Id Operation Name Rows Bytes Cost (%CPU) Time ------------------------------------------------------------------------------------ 0 SELECT STATEMENT 1 34 2 (0) 00:00:01 1 TABLE ACCESS BY INDEX ROWID T 1 34 2 (0) 00:00:01 * 2 INDEX RANGE SCAN I_T 1 1 (0) 00:00:01 ------------------------------------------------------------------------------------Predicate Information (identified by operation id): --------------------------------------------------- 2 - access("USERNAME"='EYGLE')Note ----- - dynamic sampling used for this statement 当索引字段答应为Null时,Oracle放弃此索引: SQL> alter table t modify (username null);Table altered.SQL> select /*+ index(t,i_t) */ * from t;Execution Plan ---------------------------------------------------------- Plan hash value: 1601196873-------------------------------------------------------------------------- Id Operation Name Rows Bytes Cost (%CPU) Time -------------------------------------------------------------------------- 0 SELECT STATEMENT 27 918 2 (0) 00:00:01 1 TABLE ACCESS FULL T 27 918 2 (0) 00:00:01 --------------------------------------------------------------------------Note ----- - dynamic sampling used for this statement 当该字段为Not Null时,索引可以被强制使用: SQL> alter table t modify (username not null);Table altered.SQL> select /*+ index(t,i_t) */ * from t;Execution Plan ---------------------------------------------------------- Plan hash value: 3593393735------------------------------------------------------------------------------------ Id Operation Name Rows Bytes Cost (%CPU) Time ------------------------------------------------------------------------------------ 0 SELECT STATEMENT 27 918 2 (0) 00:00:01 1 TABLE ACCESS BY INDEX ROWID T 27 918 2 (0) 00:00:01 2 INDEX FULL SCAN I_T 27 1 (0) 00:00:01 ------------------------------------------------------------------------------------Note ----- - dynamic sampling used for this statement 这就是Null值对于索引及查询的影响. 原文地址：http://www.eygle.com/archives/2006/02/index_null_hints_explain.Html |
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