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常数复合索引应用案例

原创 Linux操作系统 作者:yangtingkun 时间:2011-05-19 23:46:36 0 删除 编辑

从一个客户的真实优化案例引申的问题。

 

 

客户的一个数据库需要进行优化,不过由于程序开发方没有介入,因此这次优化无法对SQL进行修改。

仅对数据库级的调整一般来说收效不大,不过发现客户数据库中个别的SQL存在性能问题,且这个性能问题已经影响到整个数据库。如果可以将这个SQL优化,那么可以解决目前数据库的性能问题。幸运的是,这个问题可以通过添加索引来进行优化。

模拟问题SQL如下:

SQL> select * from v$version;

BANNER
----------------------------------------------------------------
Oracle9i Enterprise Edition Release 9.2.0.4.0 - Production
PL/SQL Release 9.2.0.4.0 - Production
CORE    9.2.0.3.0       Production
TNS for Linux: Version 9.2.0.4.0 - Production
NLSRTL Version 9.2.0.4.0 – Production

SQL> create table t (id number not null, created date, other char(200));

Table created.

SQL> insert into t select rownum, created, 'a' from all_objects;

31126 rows created.

SQL> commit;

Commit complete.

SQL> exec dbms_stats.gather_table_stats(user, 'T')

PL/SQL procedure successfully completed.

SQL> var v_id number
SQL> var v_date varchar2(14)
SQL> explain plan for
  2  select count(*)
  3  from t
  4  where nvl(created, sysdate) > to_date(:v_date, 'yyyymmddhh24miss') - 3
  5  and id = :v_id;

Explained.

SQL> select * from table(dbms_xplan.display);

PLAN_TABLE_OUTPUT
------------------------------------------------------------------------------------------

--------------------------------------------------------------------
| Id  | Operation            |  Name       | Rows  | Bytes | Cost  |
--------------------------------------------------------------------
|   0 | SELECT STATEMENT     |             |     1 |    13 |    92 |
|   1 |  SORT AGGREGATE      |             |     1 |    13 |       |
|*  2 |   TABLE ACCESS FULL  | T           |     1 |    13 |    92 |
--------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   2 - filter(NVL("T"."CREATED",SYSDATE@!)>TO_DATE(:Z,'yyyymmddhh24miss')-3 AND "T"."ID"=TO_NUMBER(:Z))

Note: cpu costing is off

16 rows selected.

对于这个SQL,通过索引方式优化很简单,只需要建立IDCREATED上的复合索引,就可以避免全表扫描:

SQL> create index ind_t_id_created on t (id, created);

Index created.

SQL> explain plan for
  2  select count(*)
  3  from t
  4  where nvl(created, sysdate) > to_date(:v_date, 'yyyymmddhh24miss') - 3
  5  and id = :v_id;

Explained.

SQL> select * from table(dbms_xplan.display);

PLAN_TABLE_OUTPUT
-------------------------------------------------------------------------------

--------------------------------------------------------------------------
| Id  | Operation            |  Name             | Rows  | Bytes | Cost  |
--------------------------------------------------------------------------
|   0 | SELECT STATEMENT     |                   |     1 |    13 |     2 |
|   1 |  SORT AGGREGATE      |                   |     1 |    13 |       |
|*  2 |   INDEX RANGE SCAN   | IND_T_ID_CREATED  |     1 |    13 |     2 |
--------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   2 - access("T"."ID"=TO_NUMBER(:Z))
       filter(NVL("T"."CREATED",SYSDATE@!)>TO_DATE(:Z,'yyyymmddhh24miss')-3)

Note: cpu costing is off

17 rows selected.

Oracle之所以可以选择索引扫描,是由于复合索引中CREATED列为空的记录也会被保存。由于ID列为非空,而索引不会保存所有列全为空的情况,因此CREATED为空的记录同样可以在索引中找到。

事实上,即使ID不为空,由于另一个查询条件指定了ID = :V_ID,这使得访问的记录并不包括ID为空的记录,这使得复合索引仍然可以包括这个SQL需要访问的所有数据。

不过新的疑问来了,如果查询的SQL不包含ID列的限制条件,则目前的索引不在可用:

SQL> alter table t modify id null;

Table altered.

SQL> explain plan for
  2  select count(*)
  3  from t
  4  where nvl(created, sysdate) > to_date(:v_date, 'yyyymmddhh24miss') - 3;

Explained.

SQL> select * from table(dbms_xplan.display);

PLAN_TABLE_OUTPUT

------------------------------------------------------------------------------------------

--------------------------------------------------------------------
| Id  | Operation            |  Name       | Rows  | Bytes | Cost  |
--------------------------------------------------------------------
|   0 | SELECT STATEMENT     |             |     1 |     8 |    92 |
|   1 |  SORT AGGREGATE      |             |     1 |     8 |       |
|*  2 |   TABLE ACCESS FULL  | T           |  1556 | 12448 |    92 |
--------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   2 - filter(NVL("T"."CREATED",SYSDATE@!)>TO_DATE(:Z,'yyyymmddhh24miss')-3)

Note: cpu costing is off

16 rows selected.

针对这种情况,一种方法是找一个不为空的字段做联合索引,而更省空间的方法是建立复合常数索引:

SQL> create index ind_t_created0 on t(created, 0);

Index created.

SQL> explain plan for
  2  select count(*)
  3  from t
  4  where nvl(created, sysdate) > to_date(:v_date, 'yyyymmddhh24miss') - 3;

Explained.

SQL> select * from table(dbms_xplan.display);

PLAN_TABLE_OUTPUT
-----------------------------------------------------------------------------------

-------------------------------------------------------------------------
| Id  | Operation             |  Name           | Rows  | Bytes | Cost  |
-------------------------------------------------------------------------
|   0 | SELECT STATEMENT      |                 |     1 |     8 |     4 |
|   1 |  SORT AGGREGATE       |                 |     1 |     8 |       |
|*  2 |   INDEX FAST FULL SCAN| IND_T_CREATED0  |  1556 | 12448 |     4 |
-------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   2 - filter(NVL("T"."CREATED",SYSDATE@!)>TO_DATE(:Z,'yyyymmddhh24miss')-3)

Note: cpu costing is off

16 rows selected.

最终通过建立一个包含常数的复合索引,从而避免了这个SQL的全表扫描。

 

来自 “ ITPUB博客 ” ,链接:http://blog.itpub.net/4227/viewspace-695801/,如需转载,请注明出处,否则将追究法律责任。

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