Oracle分頁查詢語句（七）

在文章的最後看一下ORDER BY STOPKEY和ORDER BY在翻頁查詢的最後幾頁的效能差異：

SQL> CREATE TABLE T AS SELECT A.* FROM DBA_OBJECTS A, DBA_USERS B, TAB;

表已建立。

SQL> SELECT COUNT(*) FROM T;

COUNT(*)
----------
458064

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

PL/SQL 過程已成功完成。

SQL> SET AUTOT TRACE
SQL> SET TIMING 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 ORDER BY TIMESTAMP
8 )
9 WHERE ROWNUM <= 20
10 )
11 WHERE RN >= 11;

已選擇10行。

經過時間: 00: 00: 00.03

Execution Plan
----------------------------------------------------------
0 SELECT STATEMENT Optimizer=CHOOSE (Cost=13888 Card=20 Bytes=1840)
1 0 VIEW (Cost=13888 Card=20 Bytes=1840)
2 1 COUNT (STOPKEY)
3 2 VIEW (Cost=13888 Card=458064 Bytes=36187056)
4 3 SORT (ORDER BY STOPKEY) (Cost=13888 Card=458064 Bytes=18780624)
5 4 TABLE ACCESS (FULL) OF 'T' (Cost=537 Card=458064 Bytes=18780624)

 

Statistics
----------------------------------------------------------
0 recursive calls
0 db block gets
5579 consistent gets
0 physical reads
0 redo size
694 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 )
10 WHERE RN BETWEEN 11 AND 20;

已選擇10行。

經過時間: 00: 00: 09.05

Execution Plan
----------------------------------------------------------
0 SELECT STATEMENT Optimizer=CHOOSE (Cost=13888 Card=458064 Bytes=42141888)
1 0 VIEW (Cost=13888 Card=458064 Bytes=42141888)
2 1 COUNT
3 2 VIEW (Cost=13888 Card=458064 Bytes=36187056)
4 3 SORT (ORDER BY) (Cost=13888 Card=458064 Bytes=18780624)
5 4 TABLE ACCESS (FULL) OF 'T' (Cost=537 Card=458064 Bytes=18780624)

 

Statistics
----------------------------------------------------------
0 recursive calls
41 db block gets
5579 consistent gets
7935 physical reads
0 redo size
689 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

對於翻頁查詢的前幾頁，採用ORDER BY STOPKEY的方式比ORDER BY效能上有很大的優勢，那麼對於分頁查詢的最後幾頁，ORDER BY STOPKEY是否和其他分頁查詢技術一樣，效能比普通方式還要低很多：

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 <= 458060
10 )
11 WHERE RN >= 458051;

已選擇10行。

經過時間: 00: 00: 09.07

Execution Plan
----------------------------------------------------------
0 SELECT STATEMENT Optimizer=CHOOSE (Cost=13888 Card=458060 Bytes=42141520)
1 0 VIEW (Cost=13888 Card=458060 Bytes=42141520)
2 1 COUNT (STOPKEY)
3 2 VIEW (Cost=13888 Card=458064 Bytes=36187056)
4 3 SORT (ORDER BY STOPKEY) (Cost=13888 Card=458064 Bytes=18780624)
5 4 TABLE ACCESS (FULL) OF 'T' (Cost=537 Card=458064 Bytes=18780624)

 

Statistics
----------------------------------------------------------
0 recursive calls
41 db block gets
5579 consistent gets
7933 physical reads
0 redo size
667 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 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 458051 AND 458060;

已選擇10行。

經過時間: 00: 00: 10.01

Execution Plan
----------------------------------------------------------
0 SELECT STATEMENT Optimizer=CHOOSE (Cost=13888 Card=458064 Bytes=42141888)
1 0 VIEW (Cost=13888 Card=458064 Bytes=42141888)
2 1 COUNT
3 2 VIEW (Cost=13888 Card=458064 Bytes=36187056)
4 3 SORT (ORDER BY) (Cost=13888 Card=458064 Bytes=18780624)
5 4 TABLE ACCESS (FULL) OF 'T' (Cost=537 Card=458064 Bytes=18780624)

 

Statistics
----------------------------------------------------------
0 recursive calls
41 db block gets
5579 consistent gets
7935 physical reads
0 redo size
649 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

出乎意料的是，雖然ORDER BY STOPKEY的方式在分頁查詢的最後幾頁效能也有明顯的下降，但是在和普通的ORDER BY相比，無論從邏輯讀、物理讀還是從執行時間上看，二者都屬於一個數量級上的。

看來ORDER BY STOPKEY排序方式，在STOPKEY接近排序總量的時候也不會有明顯的效能下降。