部分行索引使用介绍

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部分行索引使用介绍

2009/08/26 BY MACLEAN
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函数索引是Oracle索引中比较特殊的，我们这里讨论函数索引中部分行索引的使用。

部分行索引顾名思义仅就表中的一部分记录做索引，请看代码示例:

drop table test;

create table test (t1 int, t2 char(1));

declare

i int :=0;

begin

while i<100000

loop

insert into test values( i, ‘N’);

i:=i+1;

commit;

end loop;

end;

在test表上插入大量t2为N的行，并插入少量t2为Y的行

create index ind_t2y on test( case t2 when ‘Y’ then t2 end);

SQL> select count(*) from test;

COUNT(*)

———-

100004

表上供有100004条数据

SQL> select count(*) from test where t2=’Y';

COUNT(*)

———-

4

为t2列为’Y'的共有4条。

我们来分析该索引：

SQL> analyze index ind_t2y validate structure;

索引已分析

SQL> select lf_rows from index_stats;

LF_ROWS

———-

4

可以看到确实仅记录了4条记录。

我们尝试利用此部分行索引:

SQL> set autotrace on;

SQL> select count(*) from test where t2=’Y';

COUNT(*)

———-

4

Execution Plan

———————————————————-

Plan hash value: 1950795681

—————————————————————————

| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |

—————————————————————————

| 0 | SELECT STATEMENT | | 1 | 3 | 43 (12)| 00:00:01 |

| 1 | SORT AGGREGATE | | 1 | 3 | | |

|* 2 | TABLE ACCESS FULL| TEST | 2 | 6 | 43 (12)| 00:00:01 |

—————————————————————————

Predicate Information (identified by operation id):

—————————————————

2 – filter(“T2″=’Y')

Note

—–

- dynamic sampling used for this statement

Statistics

———————————————————-

0 recursive calls

0 db block gets

171 consistent gets

0 physical reads

0 redo size

515 bytes sent via SQL*Net to client

469 bytes received via SQL*Net from client

2 SQL*Net roundtrips to/from client

0 sorts (memory)

0 sorts (disk)

1 rows processed

没有如预期地使用索引，我们加上hint 再试试

SQL> select /*+ index(test ind_t2y) */ count(*) from test where t2=’Y';

COUNT(*)

———-

4

Execution Plan

———————————————————-

Plan hash value: 2501600095

—————————————————————————————-

| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |

—————————————————————————————-

| 0 | SELECT STATEMENT | | 1 | 3 | 2 (0)| 00:00:01 |

| 1 | SORT AGGREGATE | | 1 | 3 | | |

|* 2 | TABLE ACCESS BY INDEX ROWID| TEST | 2 | 6 | 2 (0)| 00:00:01 |

| 3 | INDEX FULL SCAN | IND_T2Y | 98705 | | 1 (0)| 00:00:01 |

—————————————————————————————-

Predicate Information (identified by operation id):

—————————————————

2 – filter(“T2″=’Y')

Note

—–

- dynamic sampling used for this statement

Statistics

———————————————————-

0 recursive calls

0 db block gets

2 consistent gets

0 physical reads

0 redo size

515 bytes sent via SQL*Net to client

469 bytes received via SQL*Net from client

2 SQL*Net roundtrips to/from client

0 sorts (memory)

0 sorts (disk)

1 rows processed

在使用部分行索引的情况下逻辑读大大下降了。

在不加hint的情况下优化器似乎永远无法做出正确的选择，即便修改了CBO相关的参数：

SQL> alter system set optimizer_index_cost_adj=1;

System altered.

SQL> select t2 from test where t2=’Y';

T

-

Y

Y

Y

Y

Execution Plan

———————————————————-

Plan hash value: 1357081020

————————————————————————–

| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |

————————————————————————–

| 0 | SELECT STATEMENT | | 2 | 6 | 43 (12)| 00:00:01 |

|* 1 | TABLE ACCESS FULL| TEST | 2 | 6 | 43 (12)| 00:00:01 |

————————————————————————–

仅在where 子句中指定了case when then模式时，优化器自觉地使用了该部分行索引：

SQL> select * from test where case t2 when ‘Y’ then t2 end =’Y';

T1 T

———- -

100001 Y

100002 Y

100003 Y

100004 Y

Execution Plan

———————————————————-

Plan hash value: 837354983

—————————————————————————————

| Id | Operation | Name | Rows | Bytes | Cost (%CPU)| Time |

—————————————————————————————

| 0 | SELECT STATEMENT | | 2 | 32 | 1 (0)| 00:00:01 |

| 1 | TABLE ACCESS BY INDEX ROWID| TEST | 2 | 32 | 1 (0)| 00:00:01 |

|* 2 | INDEX RANGE SCAN | IND_T2Y | 1 | | 1 (0)| 00:00:01 |

—————————————————————————————

Predicate Information (identified by operation id):

—————————————————

2 – access(CASE WHEN “T2″=’Y’ THEN “T2″ END =’Y')

Note

—–

- dynamic sampling used for this statement

Statistics

———————————————————-

0 recursive calls

0 db block gets

4 consistent gets

0 physical reads

0 redo size

650 bytes sent via SQL*Net to client

469 bytes received via SQL*Net from client

2 SQL*Net roundtrips to/from client

0 sorts (memory)

0 sorts (disk)

4 rows processed

部分行索引在特定情况下(譬如某表中仅少数特定行有大量查询更新操作)可以发挥非常巨大的作用。