ITPub博客

首页 > 应用开发 > IT综合 > 关于USE_NL的使用

关于USE_NL的使用

原创 IT综合 作者:kl911 时间:2009-12-26 09:10:55 0 删除 编辑
之前一直都不是很清楚这个怎么在Nest Loop中指定驱动表(外部表) 和 内部表,经过了一些实验对这一部分有了一定的认识:
首先对于nest loop有两种机制: 内部表全表扫描和内部表索引扫描,9i开始对于内部表索引扫描的情况有存在两种方式: 内部表索引range scan & 内部表索引uniqu scan, 索引range scan这种方式可以减少大的嵌套循环连接中的逻辑IO,减少锁存,甚至物理IO, 这个过程称为Table Prefetching.
另外对于nest loop, 驱动表和内部表位置如下:
1) 两个表全表扫描 :
NESTED LOOPS
TABLE ACCESS (FULL) OF 'Driving table'
TABLE ACCESS (FULL) OF 'Inner table'
2) Inner 表索引扫描:
NESTED LOOPS
TABLE ACCESS (FULL) OF 'Driving table'
INDEX RANGE SCAN OF 'Index of inner table'

问题一: 如果通过hint指定Nest loop的驱动表?
1) /*+ ORDERED USE_NL(a b) */ 注意这时并非a 为驱动表, b为内部表,ORDERED和Where子句的顺序相关:
drop table tmpa;
drop table tmpb;
create table tmpa as select rownum a, object_id b from DBA_objects where rownum<10000;
create table tmpb as select rownum a, object_id b from user_objects where rownum<10;

kl@k02> exec dbms_stats.gather_table_stats('kl','TMPB');

PL/SQL procedure successfully completed.

kl@k02> exec dbms_stats.gather_table_stats('kl','TMPB');

PL/SQL procedure successfully completed.

set autotrace traceonly exp stat

---- 该顺序可以看出驱动表TMPB是个小表, TMPA是个大表:
kl@k02> select /*+ use_nl(a b) */ * from tmpa a,tmpb b where a.a=b.a;

Execution Plan
----------------------------------------------------------
0 SELECT STATEMENT Optimizer=CHOOSE (Cost=38 Card=9 Bytes=144)
1 0 NESTED LOOPS (Cost=38 Card=9 Bytes=144)
2 1 TABLE ACCESS (FULL) OF 'TMPB' (Cost=2 Card=9 Bytes=72)
3 1 TABLE ACCESS (FULL) OF 'TMPA' (Cost=4 Card=1 Bytes=8)

----use_nl(a b)
kl@k02> select /*+ ordered use_nl(a b) */ * from tmpa a,tmpb b where a.a=b.a;

Execution Plan
----------------------------------------------------------
0 SELECT STATEMENT Optimizer=CHOOSE (Cost=20002 Card=9 Bytes=144)
1 0 NESTED LOOPS (Cost=20002 Card=9 Bytes=144)
2 1 TABLE ACCESS (FULL) OF 'TMPA' (Cost=4 Card=9999 Bytes=79992)
3 1 TABLE ACCESS (FULL) OF 'TMPB' (Cost=2 Card=1 Bytes=8)


----顺序并没有变
kl@k02> select /*+ ordered use_nl(b a) */ * from tmpa a,tmpb b where a.a=b.a;

Execution Plan
----------------------------------------------------------
0 SELECT STATEMENT Optimizer=CHOOSE (Cost=20002 Card=9 Bytes=144)
1 0 NESTED LOOPS (Cost=20002 Card=9 Bytes=144)
2 1 TABLE ACCESS (FULL) OF 'TMPA' (Cost=4 Card=9999 Bytes=79992)
3 1 TABLE ACCESS (FULL) OF 'TMPB' (Cost=2 Card=1 Bytes=8)

---- 看看顺序实际是安from子句的顺序
kl@k02> select /*+ ordered use_nl(b a) */ * from tmpb b,tmpa a where a.a=b.a;

Execution Plan
----------------------------------------------------------
0 SELECT STATEMENT Optimizer=CHOOSE (Cost=38 Card=9 Bytes=144)
1 0 NESTED LOOPS (Cost=38 Card=9 Bytes=144)
2 1 TABLE ACCESS (FULL) OF 'TMPB' (Cost=2 Card=9 Bytes=72)
3 1 TABLE ACCESS (FULL) OF 'TMPA' (Cost=4 Card=1 Bytes=8)


---- Leading可以选择inner table,这个地方我之前一直不清楚,目前看来Leading的表并非驱动表,而是内部表:
kl@k02> select /*+ leading(b) use_nl(a) */ * from tmpb a,tmpa b where a.a=b.a;

Execution Plan
----------------------------------------------------------
0 SELECT STATEMENT Optimizer=CHOOSE (Cost=20002 Card=9 Bytes=144)
1 0 NESTED LOOPS (Cost=20002 Card=9 Bytes=144)
2 1 TABLE ACCESS (FULL) OF 'TMPA' (Cost=4 Card=9999 Bytes=79992) --驱动表
3 1 TABLE ACCESS (FULL) OF 'TMPB' (Cost=2 Card=1 Bytes=8)



kl@k02> select /*+ leading(a) use_nl(b) */ * from tmpb a,tmpa b where a.a=b.a;

Execution Plan
----------------------------------------------------------
0 SELECT STATEMENT Optimizer=CHOOSE (Cost=38 Card=9 Bytes=144)
1 0 NESTED LOOPS (Cost=38 Card=9 Bytes=144)
2 1 TABLE ACCESS (FULL) OF 'TMPB' (Cost=2 Card=9 Bytes=72) ---驱动表
3 1 TABLE ACCESS (FULL) OF 'TMPA' (Cost=4 Card=1 Bytes=8)


问题二:
如果没有统计信息,会怎样?
kl@k02> exec dbms_stats.delete_table_stats('kl','TMPA');

PL/SQL procedure successfully completed.

kl@k02> exec dbms_stats.delete_table_stats('kl','TMPB');

PL/SQL procedure successfully completed.

kl@k02> select /*+ ordered use_nl(a b) */ * from tmpb a,tmpa b where a.a=b.a;

Execution Plan
----------------------------------------------------------
0 SELECT STATEMENT Optimizer=CHOOSE (Cost=1310 Card=1879 Bytes=97708)
1 0 NESTED LOOPS (Cost=1310 Card=1879 Bytes=97708)
2 1 TABLE ACCESS (FULL) OF 'TMPB' (Cost=2 Card=327 Bytes=8502)
3 1 TABLE ACCESS (FULL) OF 'TMPA' (Cost=4 Card=6 Bytes=156)



kl@k02> select /*+ ordered use_nl(a b) */ * from tmpb b,tmpa a where a.a=b.a;

Execution Plan
----------------------------------------------------------
0 SELECT STATEMENT Optimizer=CHOOSE (Cost=1310 Card=1879 Bytes=97708)
1 0 NESTED LOOPS (Cost=1310 Card=1879 Bytes=97708)
2 1 TABLE ACCESS (FULL) OF 'TMPB' (Cost=2 Card=327 Bytes=8502)
3 1 TABLE ACCESS (FULL) OF 'TMPA' (Cost=4 Card=6 Bytes=156)

---- 可见如果没有统计信息,执行计划都是一样的;

问题三:
如果有索引,内部表作index range scan
kl@k02> create index b_ind on tmpa (a);

Index created.

kl@k02> exec dbms_stats.gather_table_stats('kl','TMPA');

PL/SQL procedure successfully completed.

kl@k02> exec dbms_stats.gather_table_stats('kl','TMPB');

PL/SQL procedure successfully completed.

kl@k02> select /*+ USE_NL(a b) */ * from tmpb b,tmpa a where a.a=b.a;

Execution Plan
----------------------------------------------------------
0 SELECT STATEMENT Optimizer=CHOOSE (Cost=20 Card=9 Bytes=144)
1 0 TABLE ACCESS (BY INDEX ROWID) OF 'TMPA' (Cost=2 Card=1 Bytes=8)
2 1 NESTED LOOPS (Cost=20 Card=9 Bytes=144)
3 2 TABLE ACCESS (FULL) OF 'TMPB' (Cost=2 Card=9 Bytes=72)
4 2 INDEX (RANGE SCAN) OF 'B_IND' (NON-UNIQUE) (Cost=1 Card=1)
(看到index range scan,并非card=1的表是驱动表。)

综上可以理解什么驱动表,什么是内部表, 另外怎么指定驱动表,ordered hint指定from 后先出现的表为驱动表, leading hint 指定内部表。对于 HASH_JOIN也是类似,
HASH JOIN
TABLE ACCESS (FULL) OF 'TMPB' ---TMPB是HASH表
TABLE ACCESS (FULL) OF 'TMPA' ---TMPA是Prune表。

[@more@]

来自 “ ITPUB博客 ” ,链接:http://blog.itpub.net/661919/viewspace-1030000/,如需转载,请注明出处,否则将追究法律责任。

请登录后发表评论 登录
全部评论

注册时间:2008-01-03

  • 博文量
    20
  • 访问量
    318508