函数索引与统计更新
SQL> select * from v$version;
BANNER
----------------------------------------------------------------
Oracle Database 10g Enterprise Edition Release 10.2.0.4.0 - 64bi
PL/SQL Release 10.2.0.4.0 - Production
CORE 10.2.0.4.0 Production
TNS for IBM/AIX RISC System/6000: Version 10.2.0.4.0 - Productio
NLSRTL Version 10.2.0.4.0 - Production
SQL> set timing on;
SQL> create table test as select * from dba_objects ;
表已创建。
已用时间: 00: 00: 01.77
SQL> set autotrace on;
-- 查询中用到了substr函数，由于没有索引。理所当然走全表扫描TABLE ACCESS FULL
SQL> select count(1) from test
2 where substr(object_name, 1, 3)='abc';
COUNT(1)
----------
0
已用时间: 00: 00: 00.10
执行计划
----------------------------------------------------------
----------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)|
----------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 66 | 236 (2)|
| 1 | SORT AGGREGATE | | 1 | 66 | |
| 2 | TABLE ACCESS FULL| TEST | 12 | 792 | 236 (2)|
----------------------------------------------------------------
Note
-----
- 'PLAN_TABLE' is old version
统计信息
----------------------------------------------------------
48 recursive calls
0 db block gets
1114 consistent gets
1030 physical reads
0 redo size
223 bytes sent via SQL*Net to client
243 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
--创建函数索引test_idx_obj_name
SQL> create index test_idx_obj_name on TEST (substr(object_name,1,3));
索引已创建。
已用时间: 00: 00: 00.70
--创建函数索引后，执行计划走了索引，执行时间由 00: 00: 00.10 缩短到 00: 00: 00.03
SQL> select count(1) from test
2 where substr(object_name, 1, 3)='abc';
COUNT(1)
----------
0
已用时间: 00: 00: 00.03
执行计划
----------------------------------------------------------
----------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)|
----------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 66 | 1 (0)|
| 1 | SORT AGGREGATE | | 1 | 66 | |
| 2 | INDEX RANGE SCAN| TEST_IDX_OBJ_NAME | 12 | 792 | 1 (0)|
----------------------------------------------------------------------------
Note
-----
- 'PLAN_TABLE' is old version
统计信息
----------------------------------------------------------
28 recursive calls
0 db block gets
79 consistent gets
1 physical reads
0 redo size
223 bytes sent via SQL*Net to client
243 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
-- 能不能在快点呢？我们试着对该表做个统计分析，再来看结果
SQL> analyze table test compute statistics;
表已分析。
已用时间: 00: 00: 03.23
SQL> select count(1) from test
2 where substr(object_name, 1, 3)='abc';
COUNT(1)
----------
0
已用时间: 00: 00: 00.01
执行计划
----------------------------------------------------------
----------------------------------------------------------------------------
| Id | Operation | Name | Rows | Bytes | Cost (%CPU)|
----------------------------------------------------------------------------
| 0 | SELECT STATEMENT | | 1 | 21 | 1 (0)|
| 1 | SORT AGGREGATE | | 1 | 21 | |
| 2 | INDEX RANGE SCAN| TEST_IDX_OBJ_NAME | 62 | 1302 | 1 (0)|
----------------------------------------------------------------------------
Note
-----
- 'PLAN_TABLE' is old version
统计信息
----------------------------------------------------------
1 recursive calls
0 db block gets
2 consistent gets
0 physical reads
0 redo size
223 bytes sent via SQL*Net to client
243 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
1 rows processed
--统计更新后执行时间缩短到 00: 00: 00.01 ，而且consistent gets 和recursive calls 也有所下降。
说明统计更新对提高函数索引的性能是有一定的帮助的。