今天做了分页查询的性能测试,发现写一个好的分页查询语句,性能查询的差别是巨大!

建立测试表

create table test as select * from dba_objects

执行如下分页查询语句1:

set autotrace on

select * from (select t.*, rownum rn from (select * from TEST) t where rownum < 40) where rn >21

执行计划如下:仅仅六个逻辑读.

Execution Plan
----------------------------------------------------------
Plan hash value: 891187116

----------------------------------------------------------------------------
| Id | Operation           | Name | Rows | Bytes | Cost (%CPU)| Time     |
----------------------------------------------------------------------------
|   0 | SELECT STATEMENT    |      |    39 | 7410 |     2   (0)| 00:00:01 |
|* 1 | VIEW               |      |    39 | 7410 |     2   (0)| 00:00:01 |
|* 2 |   COUNT STOPKEY     |      |       |       |            |          |
|   3 |    TABLE ACCESS FULL| TEST |    39 | 3549 |     2   (0)| 00:00:01 |
----------------------------------------------------------------------------

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

1 - filter("RN">21)
2 - filter(ROWNUM<40)

Statistics
----------------------------------------------------------
          1 recursive calls
          0 db block gets
          6 consistent gets
          0 physical reads
          0 redo size
       2158 bytes sent via SQL*Net to client
        503 bytes received via SQL*Net from client
          3 SQL*Net roundtrips to/from client
          0 sorts (memory)
          0 sorts (disk)
         18 rows processed

执行如下分页查询语句2:

SELECT * FROM
(
SELECT A.*, ROWNUM RN
FROM (SELECT * FROM TEST) A
)
WHERE RN BETWEEN 21 AND 40

执行计划如下:807个逻辑读.

Execution Plan
----------------------------------------------------------
Plan hash value: 607533624

----------------------------------------------------------------------------
| Id | Operation           | Name | Rows | Bytes | Cost (%CPU)| Time     |
----------------------------------------------------------------------------
|   0 | SELECT STATEMENT    |      | 59588 |    10M|   188   (7)| 00:00:03 |
|* 1 | VIEW               |      | 59588 |    10M|   188   (7)| 00:00:03 |
|   2 |   COUNT             |      |       |       |            |          |
|   3 |    TABLE ACCESS FULL| TEST | 59588 | 5295K|   188   (7)| 00:00:03 |
----------------------------------------------------------------------------

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

1 - filter("RN"<=40 AND "RN">=21)

Statistics
----------------------------------------------------------
          1 recursive calls
          0 db block gets
        807 consistent gets
          0 physical reads
          0 redo size
       2219 bytes sent via SQL*Net to client
        503 bytes received via SQL*Net from client
          3 SQL*Net roundtrips to/from client
          0 sorts (memory)
          0 sorts (disk)
         20 rows processed

总结,对于一个高访问量的网站,如果采用第一种的分页查询,性能会有巨大的提升!

ITPub博客

分页查询性能比较

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