Oracle分页查询语句（三）

继续看查询的第二种情况，包含表连接的情况：

SQL> CREATE TABLE T AS SELECT * FROM DBA_USERS;

表已创建。

SQL> CREATE TABLE T1 AS SELECT * FROM DBA_SOURCE;

表已创建。

SQL> ALTER TABLE T ADD CONSTRAINT PK_T PRIMARY KEY (USERNAME);

表已更改。

SQL> ALTER TABLE T1 ADD CONSTRAINT FK_T1_OWNER FOREIGN KEY (OWNER)
2 REFERENCES T(USERNAME);

表已更改。

SQL> CREATE INDEX IND_T1_OWNER ON T1(NAME);

索引已创建。

SQL> EXEC DBMS_STATS.GATHER_TABLE_STATS(USER, 'T')

PL/SQL 过程已成功完成。

SQL> EXEC DBMS_STATS.GATHER_TABLE_STATS(USER, 'T1')

PL/SQL 过程已成功完成。

创建了T表和T1表，默认情况下，HASH JOIN的效率要比NESTED LOOP高很多：

SQL> SET AUTOT TRACE
SQL> SELECT * FROM T, T1 WHERE T.USERNAME = T1.OWNER;

已选择96985行。

Execution Plan
----------------------------------------------------------
0 SELECT STATEMENT Optimizer=CHOOSE (Cost=844 Card=96985 Bytes=46164860)
1 0 HASH JOIN (Cost=844 Card=96985 Bytes=46164860)
2 1 TABLE ACCESS (FULL) OF 'T' (Cost=2 Card=12 Bytes=1044)
3 1 TABLE ACCESS (FULL) OF 'T1' (Cost=826 Card=96985 Bytes=37727165)

Statistics
----------------------------------------------------------
39 recursive calls
0 db block gets
14475 consistent gets
7279 physical reads
0 redo size
37565579 bytes sent via SQL*Net to client
71618 bytes received via SQL*Net from client
6467 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
96985 rows processed

SQL> SELECT /*+ FIRST_ROWS */ * FROM T, T1 WHERE T.USERNAME = T1.OWNER;

已选择96985行。

Execution Plan
----------------------------------------------------------
0 SELECT STATEMENT Optimizer=HINT: FIRST_ROWS (Cost=97811 Card=96985 Bytes=46164860)
1 0 NESTED LOOPS (Cost=97811 Card=96985 Bytes=46164860)
2 1 TABLE ACCESS (FULL) OF 'T1' (Cost=826 Card=96985 Bytes=37727165)
3 1 TABLE ACCESS (BY INDEX ROWID) OF 'T' (Cost=1 Card=1 Bytes=87)
4 3 INDEX (UNIQUE SCAN) OF 'PK_T' (UNIQUE)

Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
117917 consistent gets
7268 physical reads
0 redo size
37565579 bytes sent via SQL*Net to client
71618 bytes received via SQL*Net from client
6467 SQL*Net roundtrips to/from client
0 sorts (memory)
0 sorts (disk)
96985 rows processed

但是如果分页查询的内层是这种连接查询的话，使用NESTED LOOP可以更快的得到前N条记录。

下面看一下这种情况下的分页查询情况：

SQL> SELECT USER_ID, USERNAME, NAME
2 FROM
3 (
4 SELECT ROWNUM RN, USER_ID, USERNAME, NAME
5 FROM
6 (
7 SELECT T.USER_ID, T.USERNAME, T1.NAME
8 FROM T, T1
9 WHERE T.USERNAME = T1.OWNER
10 )
11 WHERE ROWNUM <= 20
12 )
13 WHERE RN >= 11;

已选择10行。

Execution Plan
----------------------------------------------------------
0 SELECT STATEMENT Optimizer=CHOOSE (Cost=830 Card=20 Bytes=1200)
1 0 VIEW (Cost=830 Card=20 Bytes=1200)
2 1 COUNT (STOPKEY)
3 2 HASH JOIN (Cost=830 Card=96985 Bytes=2909550)
4 3 TABLE ACCESS (FULL) OF 'T' (Cost=2 Card=12 Bytes=132)
5 3 TABLE ACCESS (FULL) OF 'T1' (Cost=826 Card=96985 Bytes=1842715)

Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
8 consistent gets
7 physical reads
0 redo size
574 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)
0 sorts (disk)
10 rows processed

SQL> SELECT /*+ FIRST_ROWS */ USER_ID, USERNAME, NAME
2 FROM
3 (
4 SELECT ROWNUM RN, USER_ID, USERNAME, NAME
5 FROM
6 (
7 SELECT T.USER_ID, T.USERNAME, T1.NAME
8 FROM T, T1
9 WHERE T.USERNAME = T1.OWNER
10 )
11 WHERE ROWNUM <= 20
12 )
13 WHERE RN >= 11;

已选择10行。

Execution Plan
----------------------------------------------------------
0 SELECT STATEMENT Optimizer=HINT: FIRST_ROWS (Cost=97811 Card=20 Bytes=1200)
1 0 VIEW (Cost=97811 Card=20 Bytes=1200)
2 1 COUNT (STOPKEY)
3 2 NESTED LOOPS (Cost=97811 Card=96985 Bytes=2909550)
4 3 TABLE ACCESS (FULL) OF 'T1' (Cost=826 Card=96985 Bytes=1842715)
5 3 TABLE ACCESS (BY INDEX ROWID) OF 'T' (Cost=1 Card=1 Bytes=11)
6 5 INDEX (UNIQUE SCAN) OF 'PK_T' (UNIQUE)

Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
28 consistent gets
0 physical reads
0 redo size
574 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)
0 sorts (disk)
10 rows processed

看上去似乎HASH JOIN效率更高，难道上面说错了。

其实这个现象是由于这个例子的特殊性造成的。T表是根据DBA_USERS创建，这张表很小。HASH JOIN中第一步也就是第一张表的全表扫描是无法应用STOPKEY的，这就是上面提到的NESTED LOOP比HASH JOIN优势的地方。但是，这个例子中，恰好第一张表很小，对这张表的全扫描的代价极低，因此，显得HASH JOIN效率更高。但是，这不具备共性，如果两张表的大小相近，或者Oracle错误的选择了先扫描大表，则使用HASH JOIN的效率就会低得多。

SQL> SELECT USER_ID, USERNAME, NAME
2 FROM
3 (
4 SELECT ROWNUM RN, USER_ID, USERNAME, NAME
5 FROM
6 (
7 SELECT /*+ ORDERED */ T.USER_ID, T.USERNAME, T1.NAME
8 FROM T1, T
9 WHERE T.USERNAME = T1.OWNER
10 )
11 WHERE ROWNUM <= 20
12 )
13 WHERE RN >= 11;

已选择10行。

Execution Plan
----------------------------------------------------------
0 SELECT STATEMENT Optimizer=CHOOSE (Cost=951 Card=20 Bytes=1200)
1 0 VIEW (Cost=951 Card=20 Bytes=1200)
2 1 COUNT (STOPKEY)
3 2 HASH JOIN (Cost=951 Card=96985 Bytes=2909550)
4 3 TABLE ACCESS (FULL) OF 'T1' (Cost=826 Card=96985 Bytes=1842715)
5 3 TABLE ACCESS (FULL) OF 'T' (Cost=2 Card=12 Bytes=132)

Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
8585 consistent gets
7310 physical reads
0 redo size
601 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)
0 sorts (disk)
10 rows processed

通过HINT提示，让Oracle先扫描大表，这回结果就很明显了。NESTED LOOP的效果要比HASH JOIN好得多。

下面，继续比较一下两个分页操作的写法，为了使结果更具有代表性，这里都采用了FIRST_ROWS提示，让Oracle采用NESTED LOOP的方式来进行表连接：

SQL> SELECT /*+ FIRST_ROWS */ USER_ID, USERNAME, NAME
2 FROM
3 (
4 SELECT ROWNUM RN, USER_ID, USERNAME, NAME
5 FROM
6 (
7 SELECT T.USER_ID, T.USERNAME, T1.NAME
8 FROM T, T1
9 WHERE T.USERNAME = T1.OWNER
10 )
11 WHERE ROWNUM <= 20
12 )
13 WHERE RN >= 11;

已选择10行。

Execution Plan
----------------------------------------------------------
0 SELECT STATEMENT Optimizer=HINT: FIRST_ROWS (Cost=97811 Card=20 Bytes=1200)
1 0 VIEW (Cost=97811 Card=20 Bytes=1200)
2 1 COUNT (STOPKEY)
3 2 NESTED LOOPS (Cost=97811 Card=96985 Bytes=2909550)
4 3 TABLE ACCESS (FULL) OF 'T1' (Cost=826 Card=96985 Bytes=1842715)
5 3 TABLE ACCESS (BY INDEX ROWID) OF 'T' (Cost=1 Card=1 Bytes=11)
6 5 INDEX (UNIQUE SCAN) OF 'PK_T' (UNIQUE)

Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
28 consistent gets
0 physical reads
0 redo size
574 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)
0 sorts (disk)
10 rows processed

SQL> SELECT /*+ FIRST_ROWS */ USER_ID, USERNAME, NAME
2 FROM
3 (
4 SELECT ROWNUM RN, USER_ID, USERNAME, NAME
5 FROM
6 (
7 SELECT T.USER_ID, T.USERNAME, T1.NAME
8 FROM T, T1
9 WHERE T.USERNAME = T1.OWNER
10 )
11 )
12 WHERE RN BETWEEN 11 AND 20;

已选择10行。

Execution Plan
----------------------------------------------------------
0 SELECT STATEMENT Optimizer=HINT: FIRST_ROWS (Cost=97811 Card=96985 Bytes=5819100)
1 0 VIEW (Cost=97811 Card=96985 Bytes=5819100)
2 1 COUNT
3 2 NESTED LOOPS (Cost=97811 Card=96985 Bytes=2909550)
4 3 TABLE ACCESS (FULL) OF 'T1' (Cost=826 Card=96985 Bytes=1842715)
5 3 TABLE ACCESS (BY INDEX ROWID) OF 'T' (Cost=1 Card=1 Bytes=11)
6 5 INDEX (UNIQUE SCAN) OF 'PK_T' (UNIQUE)

Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
105571 consistent gets
7299 physical reads
0 redo size
574 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)
0 sorts (disk)
10 rows processed

两种写法的效率差别极大。关键仍然是是否能将STOPKEY应用到最内层查询中。

对于表连接来说，在写分页查询的时候，可以考虑增加FIRST_ROWS提示，它有助于更快的将查询结果返回。

其实，不光是表连接，对于所有的分页查询都可以加上FIRST_ROWS提示。不过需要注意的时，分页查询的目标是尽快的返回前N条记录，因此，无论是ROWNUM还是FIRST_ROWS机制都是提高前几页的查询速度，对于分页查询的最后几页，采用这些机制不但无法提高查询速度，反而会明显降低查询效率，对于这一点使用者应该做到心中有数。