Oracle分页查询语句（四）

最后的例子说明内部循环包含排序的情况：

SQL> CREATE TABLE T AS SELECT * FROM DBA_OBJECTS;

表已创建。

SQL> CREATE INDEX IND_T_OBJECT_NAME ON T (OBJECT_NAME);

索引已创建。

SQL> ALTER TABLE T MODIFY OBJECT_NAME NOT NULL;

表已更改。

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

PL/SQL 过程已成功完成。

下面进行测试包含排序操作的分页查询。可以简单的将查询分为两种不同情况，第一种排序列就是索引列，这种可以利用索引读取，第二种排序列没有索引。

第一种情况又可以细分为：完全索引扫描和通过索引扫描定位到表记录两种情况。

无论是那种情况，都可以通过索引的全扫描来避免排序的产生。看下面的例子：

SQL> SET AUTOT TRACE

SQL> SELECT OBJECT_NAME

2 FROM

3 (

4 SELECT ROWNUM RN, OBJECT_NAME

5 FROM

6 (

7 SELECT OBJECT_NAME FROM T ORDER BY OBJECT_NAME

8 )

9 WHERE ROWNUM <= 20

10 )

11 WHERE RN >= 11;

已选择10行。

Execution Plan

----------------------------------------------------------

0 SELECT STATEMENT Optimizer=CHOOSE (Cost=26 Card=20 Bytes=1580)

1 0 VIEW (Cost=26 Card=20 Bytes=1580)

2 1 COUNT (STOPKEY)

3 2 VIEW (Cost=26 Card=6361 Bytes=419826)

4 3 INDEX (FULL SCAN) OF 'IND_T_OBJECT_NAME' (NON-UNIQUE) (Cost=26 Card=6361 Bytes=108137)

Statistics

----------------------------------------------------------

0 recursive calls

0 db block gets

3 consistent gets

0 physical reads

0 redo size

576 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

这种情况下，通过索引可以完全得到查询的结果，因此可以避免表扫描的产生，而且，由于索引已经是排序过的，因此通过索引的全扫描，连排序操作都省略了。

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 ORDER BY OBJECT_NAME

8 )

9 WHERE ROWNUM <= 20

10 )

11 WHERE RN >= 11;

已选择10行。

Execution Plan

----------------------------------------------------------

0 SELECT STATEMENT Optimizer=CHOOSE (Cost=43 Card=20 Bytes=1840)

1 0 VIEW (Cost=43 Card=20 Bytes=1840)

2 1 COUNT (STOPKEY)

3 2 VIEW (Cost=43 Card=6361 Bytes=502519)

4 3 SORT (ORDER BY STOPKEY) (Cost=43 Card=6361 Bytes=133581)

5 4 TABLE ACCESS (FULL) OF 'T' (Cost=9 Card=6361 Bytes=133581)

Statistics

----------------------------------------------------------

0 recursive calls

0 db block gets

81 consistent gets

0 physical reads

0 redo size

673 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

由于不能仅仅通过索引扫描得到查询结果，这里Oracle选择了表扫描。这是由于初始化参数设置决定的。因此，建议在分页的时候使用FIRST_ROWS提示。

SQL> SELECT /*+ FIRST_ROWS */ 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 ORDER BY OBJECT_NAME

8 )

9 WHERE ROWNUM <= 20

10 )

11 WHERE RN >= 11;

已选择10行。

Execution Plan

----------------------------------------------------------

0 SELECT STATEMENT Optimizer=HINT: FIRST_ROWS (Cost=826 Card=20 Bytes=1840)

1 0 VIEW (Cost=826 Card=20 Bytes=1840)

2 1 COUNT (STOPKEY)

3 2 VIEW (Cost=826 Card=6361 Bytes=502519)

4 3 TABLE ACCESS (BY INDEX ROWID) OF 'T' (Cost=826 Card=6361 Bytes=133581)

5 4 INDEX (FULL SCAN) OF 'IND_T_OBJECT_NAME' (NON-UNIQUE) (Cost=26 Card=6361)

Statistics

----------------------------------------------------------

0 recursive calls

0 db block gets

22 consistent gets

0 physical reads

0 redo size

673 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

使用了FIRST_ROWS提示后，Oracle不需要扫描全表，而且避免了排序操作。

下面讨论最后一种情况，排序列不是索引列。这个时候排序不可避免，但是利用给出分页格式，Oracle不会对所有数据进行排序，而是只排序前N条记录。

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 ORDER BY TIMESTAMP

8 )

9 )

10 WHERE RN BETWEEN 11 AND 20;

已选择10行。

Execution Plan

----------------------------------------------------------

0 SELECT STATEMENT Optimizer=CHOOSE (Cost=64 Card=6361 Bytes=585212)

1 0 VIEW (Cost=64 Card=6361 Bytes=585212)

2 1 COUNT

3 2 VIEW (Cost=64 Card=6361 Bytes=502519)

4 3 SORT (ORDER BY) (Cost=64 Card=6361 Bytes=260801)

5 4 TABLE ACCESS (FULL) OF 'T' (Cost=9 Card=6361 Bytes=260801)

Statistics

----------------------------------------------------------

0 recursive calls

0 db block gets

81 consistent gets

0 physical reads

0 redo size

690 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 ROWNUM RN, OBJECT_ID, OBJECT_NAME

5 FROM

6 (

7 SELECT OBJECT_ID, OBJECT_NAME FROM T ORDER BY TIMESTAMP

8 )

9 WHERE ROWNUM <= 20

10 )

11 WHERE RN >= 11;

已选择10行。

Execution Plan

----------------------------------------------------------

0 SELECT STATEMENT Optimizer=CHOOSE (Cost=64 Card=20 Bytes=1840)

1 0 VIEW (Cost=64 Card=20 Bytes=1840)

2 1 COUNT (STOPKEY)

3 2 VIEW (Cost=64 Card=6361 Bytes=502519)

4 3 SORT (ORDER BY STOPKEY) (Cost=64 Card=6361 Bytes=260801)

5 4 TABLE ACCESS (FULL) OF 'T' (Cost=9 Card=6361 Bytes=260801)

Statistics

----------------------------------------------------------

0 recursive calls

0 db block gets

81 consistent gets

0 physical reads

0 redo size

690 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

观察两种不同写法的ORDER BY步骤，一个是带STOPKEY的ORDER BY，另一个不带。在大数据量需要排序的情况下，带STOPKEY的效率要比不带STOPKEY排序的效率高得多。

SQL> INSERT INTO T SELECT T.* FROM T, USER_OBJECTS;

已创建407104行。

SQL> COMMIT;

提交完成。

SQL> SELECT OBJECT_ID, OBJECT_NAME

2 FROM

3 (

4 SELECT ROWNUM RN, OBJECT_ID, OBJECT_NAME FROM

5 (

6 SELECT OBJECT_ID, OBJECT_NAME FROM T ORDER BY TIMESTAMP

7 )

8 WHERE ROWNUM <= 20

9 )

10 WHERE RN >= 11;

已选择10行。

已用时间: 00: 00: 03.78

Execution Plan

----------------------------------------------------------

0 SELECT STATEMENT Optimizer=CHOOSE (Cost=64 Card=20 Bytes=1840)

1 0 VIEW (Cost=64 Card=20 Bytes=1840)

2 1 COUNT (STOPKEY)

3 2 VIEW (Cost=64 Card=6361 Bytes=502519)

4 3 SORT (ORDER BY STOPKEY) (Cost=64 Card=6361 Bytes=260801)

5 4 TABLE ACCESS (FULL) OF 'T' (Cost=9 Card=6361 Bytes=260801)

Statistics

----------------------------------------------------------

268 recursive calls

0 db block gets

6215 consistent gets

6013 physical reads

0 redo size

740 bytes sent via SQL*Net to client

385 bytes received via SQL*Net from client

2 SQL*Net roundtrips to/from client

6 sorts (memory)

0 sorts (disk)

10 rows processed

SQL> SELECT OBJECT_ID, OBJECT_NAME

2 FROM

3 (

4 SELECT ROWNUM RN, OBJECT_ID, OBJECT_NAME FROM

5 (

6 SELECT OBJECT_ID, OBJECT_NAME FROM T ORDER BY TIMESTAMP

7 )

8 )

9 WHERE RN BETWEEN 11 AND 20;

已选择10行。

已用时间: 00: 00: 11.86

Execution Plan

----------------------------------------------------------

0 SELECT STATEMENT Optimizer=CHOOSE (Cost=64 Card=6361 Bytes=585212)

1 0 VIEW (Cost=64 Card=6361 Bytes=585212)

2 1 COUNT

3 2 VIEW (Cost=64 Card=6361 Bytes=502519)

4 3 SORT (ORDER BY) (Cost=64 Card=6361 Bytes=260801)

5 4 TABLE ACCESS (FULL) OF 'T' (Cost=9 Card=6361 Bytes=260801)

Statistics

----------------------------------------------------------

26 recursive calls

12 db block gets

6175 consistent gets

9219 physical reads

0 redo size

737 bytes sent via SQL*Net to client

385 bytes received via SQL*Net from client

2 SQL*Net roundtrips to/from client

0 sorts (memory)

1 sorts (disk)

10 rows processed

观察两个查询语句的执行时间，以及统计信息中的排序信息。对于第一个查询语句，Oracle利用了ORDER BY STOPKEY方式进行排序，排序操作只排序需要的TOP N的数据，因此排序操作放到了内存中，而对于第二个查询语句来说，进行的数据的全排序，排序数据量大，排序操作不得不在磁盘上完成，因此耗时比较多。

通过上面的例子可以看出给出的标准分页查询格式，对于包含排序的操作仍然可以在很大程度上提高分页查询性能。