Oracle分页查询格式(十一)
Oracle的分页查询语句基本上可以按照本文给出的格式来进行套用。
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这篇介绍分析函数用于分页。
Oracle分页查询格式(一):http://yangtingkun.itpub.net/post/468/100278
Oracle分页查询格式(二):http://yangtingkun.itpub.net/post/468/101703
Oracle分页查询格式(三):http://yangtingkun.itpub.net/post/468/104595
Oracle分页查询格式(四):http://yangtingkun.itpub.net/post/468/104867
Oracle分页查询格式(五):http://yangtingkun.itpub.net/post/468/107934
Oracle分页查询格式(六):http://yangtingkun.itpub.net/post/468/108677
Oracle分页查询格式(七):http://yangtingkun.itpub.net/post/468/109834
Oracle分页查询格式(八):http://yangtingkun.itpub.net/post/468/224557
Oracle分页查询格式(九):http://yangtingkun.itpub.net/post/468/224409
Oracle分页查询格式(十):http://yangtingkun.itpub.net/post/468/224823
Oracle从8i推出了分析函数,9i中分析函数进一步发展,而且已经很稳定了。
利用分析函数的功能,一样可以实现分页的功能。
首先还是构造一张大表,作为分页查询的测试表:
SQL> CREATE TABLE T AS
2 SELECT /*+ NO_MERGE(A) NO_MERGE(B) */ *
3 FROM DBA_SEQUENCES A, DBA_OBJECTS B;
表已创建。
SQL> EXEC DBMS_STATS.GATHER_TABLE_STATS(USER, 'T')
PL/SQL过程已成功完成。
SQL> SELECT COUNT(*) FROM T;
COUNT(*)
----------
4584838
SQL> SET TIMING ON
SQL> SET AUTOT ON
SQL> SELECT OBJECT_ID, OBJECT_NAME
2 FROM
3 (
4 SELECT ROWNUM RN, OBJECT_ID, OBJECT_NAME
5 FROM
6 (
7 SELECT OBJECT_ID, OBJECT_NAME FROM T
8 ORDER BY OBJECT_NAME
9 )
10 WHERE ROWNUM <= 20
11 )
12 WHERE RN >= 11;
OBJECT_ID OBJECT_NAME
---------- ------------------------------
17869 /1005bd30_LnkdConstant
17870 /1005bd30_LnkdConstant
17869 /1005bd30_LnkdConstant
17869 /1005bd30_LnkdConstant
17869 /1005bd30_LnkdConstant
17869 /1005bd30_LnkdConstant
17869 /1005bd30_LnkdConstant
17869 /1005bd30_LnkdConstant
17869 /1005bd30_LnkdConstant
17869 /1005bd30_LnkdConstant
已选择10行。
已用时间: 00: 00: 02.00
Execution Plan
----------------------------------------------------------
0 SELECT STATEMENT ptimizer=CHOOSE (Cost=34093 Card=20 Bytes=1840)
1 0 VIEW (Cost=34093 Card=20 Bytes=1840)
2 1 COUNT (STOPKEY)
3 2 VIEW (Cost=34093 Card=4584838 Bytes=362202202)
4 3 SORT (ORDER BY STOPKEY) (Cost=34093 Card=4584838 Bytes=132960302)
5 4 TABLE ACCESS (FULL) OF 'T' (Cost=9297 Card=4584838 Bytes=132960302)
Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
96666 consistent gets
56154 physical reads
0 redo size
543 bytes sent via SQL*Net to client
503 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
1 sorts (memory)
0 sorts (disk)
10 rows processed
SQL> SELECT OBJECT_ID, OBJECT_NAME
2 FROM
3 (
4 SELECT OBJECT_NAME, OBJECT_ID,
5 ROW_NUMBER() OVER(ORDER BY OBJECT_NAME) RN
6 FROM T
7 )
8 WHERE RN BETWEEN 11 AND 20;
OBJECT_ID OBJECT_NAME
---------- ------------------------------
17870 /1005bd30_LnkdConstant
17869 /1005bd30_LnkdConstant
17870 /1005bd30_LnkdConstant
17869 /1005bd30_LnkdConstant
17869 /1005bd30_LnkdConstant
17869 /1005bd30_LnkdConstant
17869 /1005bd30_LnkdConstant
17869 /1005bd30_LnkdConstant
17869 /1005bd30_LnkdConstant
17869 /1005bd30_LnkdConstant
已选择10行。
已用时间: 00: 00: 02.09
Execution Plan
----------------------------------------------------------
0 SELECT STATEMENT ptimizer=CHOOSE (Cost=34093 Card=4584838 Bytes=421805096)
1 0 VIEW (Cost=34093 Card=4584838 Bytes=421805096)
2 1 WINDOW (SORT PUSHED RANK) (Cost=34093 Card=4584838 Bytes=132960302)
3 2 TABLE ACCESS (FULL) OF 'T' (Cost=9297 Card=4584838 Bytes=132960302)
Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
96666 consistent gets
56165 physical reads
0 redo size
548 bytes sent via SQL*Net to client
503 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
1 sorts (memory)
0 sorts (disk)
10 rows processed
仔细观察标准分页SQL和利用分析函数的分页SQL,这里面有不少有意思的差别。
首先二者得到的结果是不同的。不过这其实并没有关系,因为这本身就是两个不同的SQL,而且二者的结果都是符合查询条件的。导致这个现象的原因有两个,一个是两个SQL使用的排序算法不同,另一个是排序字段不唯一。解决这个问题其实也很简单,只需要保证排序结果唯一就可以了。
如果从性能上讲,二者没有本质的差别,由于ORDER BY的列上没有索引,因此两个SQL都必须对表进行FULL TABLE SCAN。关键在于这两个SQL在能否体现出分页的特点——STOP KEY。由于用户当前执行的查询可能只是返回前100条记录,那么分页SQL就没有必要对所有的数据进行完全的排序,只需要找到最小或最大的100条记录,就可以返回结果了。在两个SQL中,都能实现这个功能,其中标准分页采用的是:SORT (ORDER BY STOPKEY);而分析函数使用的是:WINDOW (SORT PUSHED RANK)。只要具有将STOP KEY推入到排序操作内的功能,就基本上满足分页条件。
从SQL的结构上,标准分页需要3层嵌套,而分析函数只需要2层。但是并不意味分析函数减少了一层嵌套,效率就一定高于标准分页,事实上,多次测试显示,标准分页似乎还要略快一点。与错误的分页写法相比,这两个SQL的效率都是足够高的,二者之前的差别几乎可以忽略:
SQL> SELECT OBJECT_ID, OBJECT_NAME
2 FROM
3 (
4 SELECT ROWNUM RN, OBJECT_ID, OBJECT_NAME
5 FROM
6 (
7 SELECT OBJECT_ID, OBJECT_NAME FROM T
8 ORDER BY OBJECT_NAME
9 )
10 )
11 WHERE RN BETWEEN 11 AND 20;
OBJECT_ID OBJECT_NAME
---------- ------------------------------
17870 /1005bd30_LnkdConstant
17870 /1005bd30_LnkdConstant
17869 /1005bd30_LnkdConstant
17870 /1005bd30_LnkdConstant
17870 /1005bd30_LnkdConstant
17870 /1005bd30_LnkdConstant
17869 /1005bd30_LnkdConstant
17869 /1005bd30_LnkdConstant
17869 /1005bd30_LnkdConstant
17870 /1005bd30_LnkdConstant
已选择10行。
已用时间: 00: 00: 13.18
Execution Plan
----------------------------------------------------------
0 SELECT STATEMENT ptimizer=CHOOSE (Cost=34093 Card=4584838 Bytes=421805096)
1 0 VIEW (Cost=34093 Card=4584838 Bytes=421805096)
2 1 COUNT
3 2 VIEW (Cost=34093 Card=4584838 Bytes=362202202)
4 3 SORT (ORDER BY) (Cost=34093 Card=4584838 Bytes=132960302)
5 4 TABLE ACCESS (FULL) OF 'T' (Cost=9297 Card=4584838 Bytes=132960302)
Statistics
----------------------------------------------------------
0 recursive calls
50 db block gets
96666 consistent gets
96806 physical reads
0 redo size
553 bytes sent via SQL*Net to client
503 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
1 sorts (disk)
10 rows processed
上面就是采用了错误的分页写法,使得Oracle排序了所有的数据,所需的时间是正确分页写法的6倍以上。
最后将分页设置到最后的部分,检查两个SQL的性能:
SQL> SELECT OBJECT_ID, OBJECT_NAME
2 FROM
3 (
4 SELECT ROWNUM RN, OBJECT_ID, OBJECT_NAME
5 FROM
6 (
7 SELECT OBJECT_ID, OBJECT_NAME FROM T
8 ORDER BY OBJECT_NAME
9 )
10 WHERE ROWNUM <= 4584820
11 )
12 WHERE RN >= 4584811;
OBJECT_ID OBJECT_NAME
---------- ------------------------------
28423 xml-extension-type24_T
28423 xml-extension-type24_T
28423 xml-extension-type24_T
28423 xml-extension-type24_T
28423 xml-extension-type24_T
28423 xml-extension-type24_T
28423 xml-extension-type24_T
28423 xml-extension-type24_T
28423 xml-extension-type24_T
28423 xml-extension-type24_T
已选择10行。
已用时间: 00: 00: 12.92
Execution Plan
----------------------------------------------------------
0 SELECT STATEMENT ptimizer=CHOOSE (Cost=34093 Card=4584820 Bytes=421803440)
1 0 VIEW (Cost=34093 Card=4584820 Bytes=421803440)
2 1 COUNT (STOPKEY)
3 2 VIEW (Cost=34093 Card=4584838 Bytes=362202202)
4 3 SORT (ORDER BY STOPKEY) (Cost=34093 Card=4584838 Bytes=132960302)
5 4 TABLE ACCESS (FULL) OF 'T' (Cost=9297 Card=4584838 Bytes=132960302)
Statistics
----------------------------------------------------------
0 recursive calls
50 db block gets
96666 consistent gets
96810 physical reads
0 redo size
533 bytes sent via SQL*Net to client
503 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
1 sorts (disk)
10 rows processed
SQL> SELECT OBJECT_ID, OBJECT_NAME
2 FROM
3 (
4 SELECT OBJECT_NAME, OBJECT_ID,
5 ROW_NUMBER() OVER(ORDER BY OBJECT_NAME) RN
6 FROM T
7 )
8 WHERE RN BETWEEN 4584811 AND 4584820;
OBJECT_ID OBJECT_NAME
---------- ------------------------------
28423 xml-extension-type24_T
28423 xml-extension-type24_T
28423 xml-extension-type24_T
28423 xml-extension-type24_T
28423 xml-extension-type24_T
28423 xml-extension-type24_T
28423 xml-extension-type24_T
28423 xml-extension-type24_T
28423 xml-extension-type24_T
28423 xml-extension-type24_T
已选择10行。
已用时间: 00: 00: 18.78
Execution Plan
----------------------------------------------------------
0 SELECT STATEMENT ptimizer=CHOOSE (Cost=34093 Card=4584838 Bytes=421805096)
1 0 VIEW (Cost=34093 Card=4584838 Bytes=421805096)
2 1 WINDOW (SORT PUSHED RANK) (Cost=34093 Card=4584838 Bytes=132960302)
3 2 TABLE ACCESS (FULL) OF 'T' (Cost=9297 Card=4584838 Bytes=132960302)
Statistics
----------------------------------------------------------
0 recursive calls
48 db block gets
96666 consistent gets
76497 physical reads
0 redo size
533 bytes sent via SQL*Net to client
503 bytes received via SQL*Net from client
2 SQL*Net roundtrips to/from client
0 sorts (memory)
1 sorts (disk)
10 rows processed
由于表中一个只有4584838条记录,因此这个分页基本上是最后一两页了,这可以说是一个极端的例子了,比较二者的效率发现,标准分页效率这次要比分析函数高将近50%。虽然例子比较极端,不过也可以说明一些问题。首先,通过ORDER BY加ROWNUM方式的排序算法,应该和分析函数的不同。其次,分析函数的功能很强大,ORDER BY只是其中一个功能,分析函数还能完成分区和窗口操作等更加复杂的操作,因此效率比单纯的排序要低也无可厚非。当然,为了和前面的测试保持版本的一致性,测试在920上进行,不排除10g或11g对分析函数的分页做了进一步的优化。
最后还需要提一句,采用分析函数的方法,不能没有ORDER BY语句,而标准分页方式可以。当然没有ORDER BY能不能算一个真正的分页,就是见仁见智的事情了。
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