Oracle中怎么使用NESTED LOOP操作

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通過例子說明分頁查詢使用的NESTED LOOP操作，在分頁查詢翻到最后幾頁時的性能問題：

SQL> CREATE TABLE T AS SELECT * FROM DBA_USERS;

表已創(chuàng)建。

SQL> CREATE TABLE T1 AS SELECT * FROM DBA_SOURCE;

表已創(chuàng)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);

索引已創(chuàng)建。

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

對于最后幾頁，采用HASH JOIN的方式，執(zhí)行效率幾乎沒有任何改變，而采用NESTED LOOP方式，則效率嚴重下降，而且遠遠低于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，很少會翻到最后一篇，如果只是偶爾碰到這種情況，對系統(tǒng)性能不會有很大的影響，但是如果經(jīng)常碰到這種情況，在設計分頁查詢時應該給予足夠的考慮。

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