Oracle分頁查詢語句（六）

這篇文章通過例子說明分頁查詢使用的NESTED LOOP操作，在分頁查詢翻到最後幾頁時的效能問題：

SQL> CREATE TABLE T AS SELECT * FROM DBA_USERS;

表已建立。

SQL> CREATE TABLE T1 AS SELECT * FROM DBA_SOURCE;

表已建立。

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);

索引已建立。

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

PL/SQL 過程已成功完成。

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

PL/SQL 過程已成功完成。

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;

已選擇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 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;

已選擇10行。

Execution Plan
----------------------------------------------------------
0 SELECT STATEMENT Optimizer=CHOOSE (Cost=830 Card=96985 Bytes=5819100)
1 0 VIEW (Cost=830 Card=96985 Bytes=5819100)
2 1 COUNT
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
8586 consistent gets
8052 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

在分頁查詢的前幾頁，NESTED LOOP操作比HASH JOIN操作效率高得多。

SQL> SET AUTOT OFF
SQL> SELECT COUNT(*) FROM T, T1 WHERE USERNAME = OWNER;

COUNT(*)
----------
96985

SQL> SET AUTOT TRACE

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 )
12 WHERE RN BETWEEN 96971 AND 96980;

已選擇10行。

Execution Plan
----------------------------------------------------------
0 SELECT STATEMENT Optimizer=CHOOSE (Cost=830 Card=96985 Bytes=5819100)
1 0 VIEW (Cost=830 Card=96985 Bytes=5819100)
2 1 COUNT
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
8586 consistent gets
8068 physical reads
0 redo size
571 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

對於最後幾頁，採用HASH JOIN的方式，執行效率幾乎沒有任何改變，而採用NESTED LOOP方式，則效率嚴重下降，而且遠遠低於HASH JOIN的方式。

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 <= 96980
12 )
13 WHERE RN >= 96971;

已選擇10行。

Execution Plan
----------------------------------------------------------
0 SELECT STATEMENT Optimizer=HINT: FIRST_ROWS (Cost=97811 Card=96980 Bytes=5818800)
1 0 VIEW (Cost=97811 Card=96980 Bytes=5818800)
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
105566 consistent gets
8068 physical reads
0 redo size
571 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

分頁查詢一般情況下，很少會翻到最後一篇，如果只是偶爾碰到這種情況，對系統效能不會有很大的影響，但是如果經常碰到這種情況，在設計分頁查詢時應該給予足夠的考慮。