SQL hint中正确使用use_nl提示

[导读]之前对use_nl的理解一直很模糊，看下面的案例。SQL> select * 2 from table_detail t, 3 table(cast(str2varlist('1

之前对use_nl的理解一直很模糊，看下面的案例。

SQL> select *
2 from table_detail t,
3 table(cast(str2varlist('123') as vartabletype)) t2
4 where t.id = t2.column_value;

Execution Plan
----------------------------------------------------------
Plan hash value: 31598426
----------------------------------------------------------
Operation | Name |Rows |Bytes|Cost(%CPU)|Time|
----------------------------------------------------------
SELECT STATEMENT | |784K|124M|2132(1)|00:00:26|
NESTED LOOPS | |784K|124M|2132(1)|00:00:26|
TABLE ACCESS FULL | table_detail|96 |15744| 11(0)|00:00:01|
COLLECTION ITERATOR PICKLER FETCH| STR2VARLIST | | | | |
-------------------------------------------------------------

t2是个虚拟的表，没有准确的统计信息，很容易出现执行计划错误，这种sql最好要用hint来固化其执行计划。

1.使用/*+ ORDERED use_nl(t2,t) */提示

SQL> select /*+ ORDERED use_nl(t2,t) */*
2 from table_detail t,
3 table(cast(str2varlist('123') as vartabletype)) t2
4 where t.id = t2.column_value;

Execution Plan
----------------------------------------------------------
Plan hash value: 3775534098

----------------------------------------------------------------------------
Operation | Name |Rows |Bytes|Cost(%CPU)|Time|
----------------------------------------------------------------------------
SELECT STATEMENT | |784K|124M|2132(1)|00:00:26|
NESTED LOOPS | |784K|124M|2132(1)|00:00:26|
TABLE ACCESS FULL | table_detail|96 |15744| 11(0)|00:00:01|
COLLECTION ITERATOR PICKLER FETCH| STR2VARLIST | | | | |
----------------------------------------------------------------------------

执行计划是错误的，还是取table_detail为驱动表。
错误理解：使用order提示，执行计划会去取use_nl(t2,t)中的t2作为驱动表。
2.使用/*+ ORDERED use_nl(t,t2) */提示

SQL> select /*+ ORDERED use_nl(t,t2) */*
2 from table_detail t,
3 table(cast(str2varlist('123') as vartabletype)) t2
4 where t.id = t2.column_value;

Execution Plan
----------------------------------------------------------
Plan hash value: 3775534098

----------------------------------------------------------------------------
Operation | Name |Rows |Bytes|Cost(%CPU)|Time|
----------------------------------------------------------------------------
SELECT STATEMENT | |784K|124M|2132(1)|00:00:26|
NESTED LOOPS | |784K|124M|2132(1)|00:00:26|
TABLE ACCESS FULL | table_detail|96 |15744| 11(0)|00:00:01|
COLLECTION ITERATOR PICKLER FETCH| STR2VARLIST | | | | |
----------------------------------------------------------------------------

执行计划还是错误的，这到底是怎么回事？
3.使用/*+ ORDERED use_nl(t,t2) */提示，同时调整了表顺序。

SQL> select /*+ ORDERED use_nl(t2,t) */*
2 from table(cast(str2varlist('123') as vartabletype)) t2,
3 table_detail t
4 where t.id = t2.column_value;

Execution Plan
----------------------------------------------------------
Plan hash value: 2272521841

-------------------------------------------------------------------------------
Operation | Name Rows |Bytes |Cost (%CPU)|Time|
-------------------------------------------------- ----------------------------
SELECT STATEMENT | |784K| 124M|25 (0)|00:00:01|
TABLE ACCESS BY INDEX ROWID | table_detail | 96 |15744| 1 (0)|00:00:01|
NESTED LOOPS | |784K| 124M|25 (0)|00:00:01|
COLLECTION ITERATOR PICKLER FETCH| STR2VARLIST | | | | |
INDEX RANGE SCAN | IND_deail_id | 1 | | 1 (0)|00:00:01|
-------------------------------------------------- ----------------------------

这回执行计划是正确的，原来order是根据from表顺序来决定驱动表，而不是use_nl(t,t2)中表的先后顺序。
调整from后面的表顺序是很老土的调优，仿佛回到了rbo的年代，采用leading指定驱动表看是否可行。
4.使用leading提示来指定驱动表

SQL> select /*+ leading(t2) use_nl(t) */*
2 from table_detail t,
3 table(cast(str2varlist('123') as vartabletype)) t2
4 where t.id = t2.column_value;

Execution Plan
----------------------------------------------------------
Plan hash value: 2272521841
-------------------------------------------------------------------------------
Operation | Name Rows |Bytes |Cost (%CPU)|Time|
-------------------------------------------------- ----------------------------
SELECT STATEMENT | |784K| 124M|25 (0)|00:00:01|
TABLE ACCESS BY INDEX ROWID | table_detail | 96 |15744| 1 (0)|00:00:01|
NESTED LOOPS | |784K| 124M|25 (0)|00:00:01|
COLLECTION ITERATOR PICKLER FETCH| STR2VARLIST | | | | |
INDEX RANGE SCAN | IND_deail_id | 1 | | 1 (0)|00:00:01|
-------------------------------------------------- ----------------------------

1./*+use_nl(t2,t) */ 提示走nest loop,但没有提示t2还是t为驱动表。
2./*+ ordered use_nl(t2,t) */提示走nest loop，order提示的是from后面的第一个表为驱动表。
3./*+ leading(t2) use_nl(t) */ 直接提示t2为驱动表。
结论：use_nl不能让优化器确定谁是驱动表谁是被驱动的表,use_nl(t,t2)也没有指出哪个是驱动表，这时候我们需要使用ordered,leading来强制指定驱动表，以达到我们的目的。