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。