分页语句的快与慢

今天又有人在问分页语句的问题：为什么最后的分页会慢。

这儿我们通过执行计划来分析一下，首先建立测试用表。

drop table test1 purge;
create table test1 as select * from dba_objects;
SQL> select count(*) from test1;

COUNT(*)
----------
87028

先用autotrace来看下plan

SQL> set autotrace traceonly
SELECT object_id, object_name
FROM (SELECT rownum AS sn, a.*
FROM (SELECT object_id, object_name FROM test1 ORDER BY object_id) a
WHERE rownum <= 100) a
WHERE sn >= 90;
Elapsed: 00:00:00.34

Execution Plan
----------------------------------------------------------
Plan hash value: 3077351352

-------------------------------------------------------------------------------------------
| Id  | Operation		       | Name	  | Rows  | Bytes | Cost (%CPU)| Time	  |
-------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT	       |	  |   100 |  9200 |	4   (0)| 00:00:01 |
|*  1 |  VIEW			       |	  |   100 |  9200 |	4   (0)| 00:00:01 |
|*  2 |   COUNT STOPKEY 	       |	  |	  |	  |	       |	  |
|   3 |    VIEW 		       |	  | 90705 |  6997K|	4   (0)| 00:00:01 |
|   4 |     TABLE ACCESS BY INDEX ROWID| TEST1	  | 90705 |  6997K|	4   (0)| 00:00:01 |
|   5 |      INDEX FULL SCAN	       | PK_TEST1 |   101 |	  |	2   (0)| 00:00:01 |
-------------------------------------------------------------------------------------------

先不忙下结论，我们再用display_corsor来看下执行计划

SQL> set autotrace off
SQL> /

OBJECT_ID OBJECT_NAME
---------- --------------------------------------------------------------------------------------------------------------------------------
91 I_NTAB3
92 REFCON$
93 I_REFCON1
94 I_REFCON2
95 OPQTYPE$
96 I_OPQTYPE1
97 UGROUP_SEQUENCE
98 PROPS$
99 EDITION$
100 ORA$BASE
101 FIXED_OBJ$
Elapsed: 00:00:00.04
SQL> select * from table(dbms_xplan.display_cursor(sql_id =>null,cursor_child_no =>0,format =>'ALL ALLSTATS LAST NOTE ADVANCED'));

PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
SQL_ID	5tjrjx4kzqbzz, child number 1
-------------------------------------
SELECT object_id, object_name	FROM (SELECT rownum AS sn, a.*
FROM (SELECT object_id, object_name FROM test1 ORDER BY object_id) a
WHERE rownum <= 100) a  WHERE sn >= 90

Plan hash value: 3077351352

-------------------------------------------------------------------------------------------------------------------------------------
| Id  | Operation		       | Name	  | Starts | E-Rows |E-Bytes| Cost (%CPU)| E-Time   | A-Rows |	 A-Time   | Buffers |
-------------------------------------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT	       |	  |	 1 |	    |	    |	  4 (100)|	    |	  11 |00:00:00.02 |	  6 |
|*  1 |  VIEW			       |	  |	 1 |	100 |  9200 |	  4   (0)| 00:00:01 |	  11 |00:00:00.02 |	  6 |
|*  2 |   COUNT STOPKEY 	       |	  |	 1 |	    |	    |		 |	    |	 100 |00:00:00.02 |	  6 |
|   3 |    VIEW 		       |	  |	 1 |  90705 |  6997K|	  4   (0)| 00:00:01 |	 100 |00:00:00.01 |	  6 |
|   4 |     TABLE ACCESS BY INDEX ROWID| TEST1	  |	 1 |  90705 |  6997K|	  4   (0)| 00:00:01 |	 100 |00:00:00.01 |	  6 |
|   5 |      INDEX FULL SCAN	       | PK_TEST1 |	 1 |	101 |	    |	  2   (0)| 00:00:01 |	 100 |00:00:00.01 |	  3 |
-------------------------------------------------------------------------------------------------------------------------------------

有细心的读者或许会发现两个plan的不同。

第二个里面A-Rows只有100，这说明两个问题：

1、分页语句不要用autotrace看plan。

2、分页语句读取了100行后便返回了，所以前面几页一般都比较快。

那么后面的几页呢？

看下面的语句及plan

SELECT object_id, object_name
FROM (SELECT rownum AS sn, a.*
FROM (SELECT object_id, object_name FROM test1 ORDER BY object_id) a
WHERE rownum <= 80100) a
5   WHERE sn >= 80090;

OBJECT_ID OBJECT_NAME
---------- --------------------------------------------------------------------------------------------------------------------------------
80938 STATUS_REC
80939 STATUS_REC_ARR
80940 AUDIT_OBJ
80941 AUDIT_ARRAY
80942 MGMT_AUDIT_CUSTOM_ATTRIB_OBJ
80943 MGMT_AUDIT_CUSTOM_ATTRIB_ARRAY
80944 MGMTCHRNSREGIONMEMBERS
80945 MGMTCHRNSEVENTURLINFO
80946 MGMTCHRNSEVENTURLINFOTABLE
80947 MGMTCHRNSTGTGUID
80948 MGMTCHRNSTGTGUIDS
Elapsed: 00:00:10.43
SQL> select * from table(dbms_xplan.display_cursor(sql_id =>null,cursor_child_no =>0,format =>'ALL ALLSTATS LAST NOTE ADVANCED'));

PLAN_TABLE_OUTPUT
--------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
SQL_ID	9gpabx9hxwpc0, child number 0
-------------------------------------
SELECT object_id, object_name	FROM (SELECT rownum AS sn, a.*
FROM (SELECT object_id, object_name FROM test1 ORDER BY object_id) a
WHERE rownum <= 80100) a  WHERE sn >= 80090

Plan hash value: 3077351352

-------------------------------------------------------------------------------------------------------------------------------------
| Id  | Operation		       | Name	  | Starts | E-Rows |E-Bytes| Cost (%CPU)| E-Time   | A-Rows |	 A-Time   | Buffers |
-------------------------------------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT	       |	  |	 1 |	    |	    |  1357 (100)|	    |	  11 |00:00:10.08 |    1368 |
|*  1 |  VIEW			       |	  |	 1 |  80100 |  7196K|  1357   (1)| 00:00:17 |	  11 |00:00:10.08 |    1368 |
|*  2 |   COUNT STOPKEY 	       |	  |	 1 |	    |	    |		 |	    |  80100 |00:00:08.73 |    1368 |
|   3 |    VIEW 		       |	  |	 1 |  90705 |  6997K|  1357   (1)| 00:00:17 |  80100 |00:00:06.32 |    1368 |
|   4 |     TABLE ACCESS BY INDEX ROWID| TEST1	  |	 1 |  90705 |  6997K|  1357   (1)| 00:00:17 |  80100 |00:00:03.86 |    1368 |
|   5 |      INDEX FULL SCAN	       | PK_TEST1 |	 1 |  80101 |	    |	173   (0)| 00:00:03 |  80100 |00:00:01.23 |	168 |
-------------------------------------------------------------------------------------------------------------------------------------

可以看到A-Rows为80100，几乎访问该表所有的行。

访问的行数差异大，速度差异大也就相当正常了。