在不少时候我们需要对一个值是否是null的查询,根据Oracle的特点,我们单纯在在这个列上创建一个index不能满足这个需求,因为b-tree index中就是不包含null列.通过创建含常数列的复合index可以满足该需求
数据库版本
SQL> select * from v$version; BANNER ---------------------------------------------------------------- Oracle Database 10g Enterprise Edition Release 10.2.0.4.0 - Prod PL/SQL Release 10.2.0.4.0 - Production CORE 10.2.0.4.0 Production TNS for Linux: Version 10.2.0.4.0 - Production NLSRTL Version 10.2.0.4.0 - Production
创建测试表
SQL> create table t_xifenfei 2 as 3 select * from dba_objects; Table created. SQL> desc t_xifenfei; Name Null? Type ----------------------------------------- -------- ---------------------------- OWNER VARCHAR2(30) OBJECT_NAME VARCHAR2(128) SUBOBJECT_NAME VARCHAR2(30) OBJECT_ID NUMBER DATA_OBJECT_ID NUMBER OBJECT_TYPE VARCHAR2(19) CREATED DATE LAST_DDL_TIME DATE TIMESTAMP VARCHAR2(19) STATUS VARCHAR2(7) TEMPORARY VARCHAR2(1) GENERATED VARCHAR2(1) SECONDARY VARCHAR2(1)
创建可能含null列index
SQL> create index ind_object_id on t_xifenfei(object_id); Index created. SQL> exec dbms_stats.gather_table_stats(user,'T_XIFENFEI',cascade => true); PL/SQL procedure successfully completed.
查看执行计划
SQL> SET AUTOT TRACE EXP stat
SQL> SELECT * FROM T_XIFENFEI WHERE OBJECT_ID IS NULL;
no rows selected
Execution Plan
----------------------------------------------------------
Plan hash value: 548923532
--------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
--------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 95 | 159 (2)| 00:00:02 |
|* 1 | TABLE ACCESS FULL| T_XIFENFEI | 1 | 95 | 159 (2)| 00:00:02 |
--------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
1 - filter("OBJECT_ID" IS NULL)
Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
695 consistent gets
0 physical reads
0 redo size
995 bytes sent via SQL*Net to client
389 bytes received via SQL*Net from client
1 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
0 rows processed
可以看到我们创建的一个普通的index,查询判断是否为null的时候,没有被应用该index,而是直接使用全表扫描.
创建支持null index
SQL> drop index ind_object_id ; Index dropped. SQL> create index ind_object_id on t_xifenfei(object_id,0); Index created. SQL> exec dbms_stats.gather_table_stats(user,'T_XIFENFEI',cascade => true); PL/SQL procedure successfully completed.
查看执行计划
SQL> SELECT * FROM T_XIFENFEI WHERE OBJECT_ID IS NULL;
Execution Plan
----------------------------------------------------------
Plan hash value: 804765899
---------------------------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |
---------------------------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 95 | 1 (0)| 00:00:01 |
| 1 | TABLE ACCESS BY INDEX ROWID| T_XIFENFEI | 1 | 95 | 1 (0)| 00:00:01 |
|* 2 | INDEX RANGE SCAN | IND_OBJECT_ID | 1 | | 1 (0)| 00:00:01 |
---------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - access("OBJECT_ID" IS NULL)
Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
2 consistent gets
0 physical reads
0 redo size
995 bytes sent via SQL*Net to client
389 bytes received via SQL*Net from client
1 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
0 rows processed
这里使用了index,并且执行中逻辑读大幅度下降,很大程度提高了程序的执行效率,逻辑读从695降低为2.
原因分析
建立一个包含列和常数的复合index,可以实现该需求,根据b-tree index的特点,只有当index中包含的列都为null的时候,才不会别在index中记录,因为设置了index中包含的常数列,所以就是列为null,也会被包含在该index中,从而查询null值的时候依然可以使用到该index