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35.读书笔记收获不止Oracle之嵌套循环表连接与索引

发布时间：2020-12-12 14:16:28 所属栏目：百科 来源：网络整理
导读：35.读书笔记收获不止Oracle之嵌套循环表连接与索引 表连接的研究中，索引是非常重要的一部分，对提升表连接性能起到至关重要的作用。 1. 嵌套循环与索引 SQLalter session set statistics_level=all; SQLselect /*+leading(t1) use_nl(t2)*/ * from t1,t2 wh

35.读书笔记收获不止Oracle之嵌套循环表连接与索引

表连接的研究中，索引是非常重要的一部分，对提升表连接性能起到至关重要的作用。

1. 嵌套循环与索引

SQL>alter session set statistics_level=all;

SQL>select /*+leading(t1) use_nl(t2)*/ * from t1,t2 wheret1.id=t2.t1_id and t1.n=19;

SQL> select * from table(dbms_xplan.display_cursor(null,null,'allstatslast'));

PLAN_TABLE_OUTPUT

--------------------------------------------------------------------------------

SQL_ID 2w8kmgu3tmxhq,child number 0

-------------------------------------

select /*+leading(t1) use_nl(t2)*/ * fromt1,t2 where t1.id=t2.t1_id

and t1.n=19

Plan hash value: 1967407726

--------------------------------------------------------------------------------

-----

| Id| Operation | Name | Starts | E-Rows | A-Rows | A-Time | Buff

PLAN_TABLE_OUTPUT

--------------------------------------------------------------------------------

ers |

--------------------------------------------------------------------------------

-----

| 0| SELECT STATEMENT | | 1 | | 1 |00:00:00.01 | 1

014 |

| 1| NESTED LOOPS | | 1 | 1 | 1 |00:00:00.01 | 1

014 |

PLAN_TABLE_OUTPUT

--------------------------------------------------------------------------------

|* 2| TABLE ACCESS FULL| T1 | 1 | 1 | 1 |00:00:00.01 |

8 |

|* 3| TABLE ACCESS FULL| T2 | 1 | 1 | 1 |00:00:00.01 | 1

006 |

--------------------------------------------------------------------------------

-----

Predicate Information (identified byoperation id):

PLAN_TABLE_OUTPUT

--------------------------------------------------------------------------------

---------------------------------------------------

2- filter("T1"."N"=19)

3- filter("T1"."ID"="T2"."T1_ID")

22 rows selected.

1.1不用HINT

SQL>select * from t1,t2 where t1.id=t2.t1_id and t1.n=19;

SQL> select * fromtable(dbms_xplan.display_cursor(null,'allstats last'));

--------------------------------------------------------------------------------

SQL_ID g7rb3y8bmguur,child number 0

-------------------------------------

select * from t1,t2 where t1.id=t2.t1_idand t1.n=19

Plan hash value: 1838229974

--------------------------------------------------------------------------------

--------------------------------

| Id| Operation | Name | Starts | E-Rows | A-Rows | A-Time | Buff

ers |OMem | 1Mem | Used-Mem |

PLAN_TABLE_OUTPUT

--------------------------------------------------------------------------------

--------------------------------------------------------------------------------

--------------------------------

| 0| SELECT STATEMENT | | 1 | | 1 |00:00:00.06 | 1

013 | | | |

|* 1| HASH JOIN | | 1 | 1 | 1 |00:00:00.06 | 1

013 | 960K| 960K|385K (0)|

|* 2| TABLE ACCESS FULL| T1 | 1 | 1 | 1 |00:00:00.01 |

PLAN_TABLE_OUTPUT

--------------------------------------------------------------------------------

7 | | | |

| 3| TABLE ACCESS FULL| T2 | 1 | 100K| 100K|00:00:00.01 | 1

006 | | | |

--------------------------------------------------------------------------------

--------------------------------

Predicate Information (identified byoperation id):

---------------------------------------------------

PLAN_TABLE_OUTPUT

--------------------------------------------------------------------------------

1- access("T1"."ID"="T2"."T1_ID")

2- filter("T1"."N"=19)

21 rows selected.

没有HINT，就走Hash Join。其实两者的消耗应该是差不多的。

1.2T1表键索引后

SQL>create index t1_n on t1(n);

SQL> select /*+leading(t1) use_nl(t2)*/ * from t1,t2 wheret1.id=t2.t1_id and t1.n=19;

SQL> select * fromtable(dbms_xplan.display_cursor(null,'allstats last'));

PLAN_TABLE_OUTPUT

--------------------------------------------------------------------------------

SQL_ID 4n9w3nnj61xk0,t2 where t1.id=t2.t1_id

and t1.n=19

Plan hash value: 2987075831

--------------------------------------------------------------------------------

-----------------------

| Id| Operation | Name | Starts | E-Rows | A-Rows |

PLAN_TABLE_OUTPUT

--------------------------------------------------------------------------------

A-Time | Buffers |

--------------------------------------------------------------------------------

-----------------------

| 0| SELECT STATEMENT | | 1 | |1 |

00:00:00.01 | 1009 |

| 1| NESTED LOOPS | | 1 | 1 |1 |

00:00:00.01 | 1009 |

PLAN_TABLE_OUTPUT

--------------------------------------------------------------------------------

| 2| TABLE ACCESS BY INDEX ROWID BATCHED|T1 | 1 | 1 |1 |

00:00:00.01 | 3 |

|* 3| INDEX RANGE SCAN | T1_N | 1 | 1 |1 |

00:00:00.01 | 2 |

|* 4| TABLE ACCESS FULL | T2| 1 | 1 |1 |

00:00:00.01 | 1006 |

--------------------------------------------------------------------------------

-----------------------

PLAN_TABLE_OUTPUT

--------------------------------------------------------------------------------

Predicate Information (identified byoperation id):

---------------------------------------------------

3- access("T1"."N"=19)

4- filter("T1"."ID"="T2"."T1_ID")

23 rows selected.

22 rows selected.

T1表走的是索引了。

1.3T2表建索引

SQL>create index t2_t1_id on t2(t1_id);

SQL> select /*+leading(t1) use_nl(t2)*/ * from t1,t2 wheret1.id=t2.t1_id and t1.n=19;

SQL> select * fromtable(dbms_xplan.display_cursor(null,'allstats last'));

PLAN_TABLE_OUTPUT

--------------------------------------------------------------------------------

SQL_ID 2w8kmgu3tmxhq,t2 where t1.id=t2.t1_id

and t1.n=19

Plan hash value: 342856344

--------------------------------------------------------------------------------

-------------------------------------

| Id| Operation | Name | Starts | E-Rows | A-R

PLAN_TABLE_OUTPUT

--------------------------------------------------------------------------------

ows | A-Time | Buffers | Reads |

--------------------------------------------------------------------------------

-------------------------------------

| 0| SELECT STATEMENT | | 1| |

1|00:00:00.03 | 7 | 4 |

| 1| NESTED LOOPS | | 1| 1 |

1|00:00:00.03 | 7 | 4 |

PLAN_TABLE_OUTPUT

--------------------------------------------------------------------------------

| 2| NESTED LOOPS | | 1| 1 |

1|00:00:00.03 | 6 | 4 |

| 3| TABLE ACCESS BY INDEX ROWID BATCHED|T1 | 1 | 1 |

1|00:00:00.01 | 3 | 0 |

|* 4| INDEX RANGE SCAN | T1_N | 1 | 1 |

1|00:00:00.01 | 2 | 0 |

|* 5| INDEX RANGE SCAN | T2_T1_ID | 1 | 1 |

1|00:00:00.03 | 3 | 4 |

PLAN_TABLE_OUTPUT

--------------------------------------------------------------------------------

| 6| TABLE ACCESS BY INDEX ROWID |T2 | 1 | 1 |

1|00:00:00.01 | 1 | 0 |

--------------------------------------------------------------------------------

-------------------------------------

Predicate Information (identified byoperation id):

---------------------------------------------------

PLAN_TABLE_OUTPUT

--------------------------------------------------------------------------------

4- access("T1"."N"=19)

5- access("T1"."ID"="T2"."T1_ID")

25 rows selected.

T2表也走索引了。现在不用HINT，ORACLE也会自己走索引了，因为代价已经很低了。

如下：

SQL>select * from t1,'allstats last'));

PLAN_TABLE_OUTPUT

--------------------------------------------------------------------------------

SQL_ID g7rb3y8bmguur,t2 where t1.id=t2.t1_idand t1.n=19

Plan hash value: 342856344

--------------------------------------------------------------------------------

----------------------------

| Id| Operation | Name | Starts | E-Rows | A-R

ows | A-Time | Buffers |

PLAN_TABLE_OUTPUT

--------------------------------------------------------------------------------

--------------------------------------------------------------------------------

----------------------------

| 0| SELECT STATEMENT | | 1| |

1|00:00:00.01 | 6 |

| 1| NESTED LOOPS | | 1| 1 |

1|00:00:00.01 | 6 |

| 2| NESTED LOOPS | | 1| 1 |

PLAN_TABLE_OUTPUT

--------------------------------------------------------------------------------

1|00:00:00.01 | 5 |

| 3| TABLE ACCESS BY INDEX ROWID BATCHED|T1 | 1 | 1 |

1|00:00:00.01 | 2 |

|* 4| INDEX RANGE SCAN | T1_N | 1 | 1 |

1|00:00:00.01 | 1 |

|* 5| INDEX RANGE SCAN | T2_T1_ID | 1 | 1 |

1|00:00:00.01 | 3 |

PLAN_TABLE_OUTPUT

--------------------------------------------------------------------------------

| 6| TABLE ACCESS BY INDEX ROWID |T2 | 1 | 1 |

1|00:00:00.01 | 1 |

--------------------------------------------------------------------------------

----------------------------

Predicate Information (identified byoperation id):

---------------------------------------------------

4- access("T1"."N"=19)

PLAN_TABLE_OUTPUT

--------------------------------------------------------------------------------

5- access("T1"."ID"="T2"."T1_ID")

Note

-----

-this is an adaptive plan

28 rows selected.

2. 最实用NL连接的场景

两表关联返回的记录不多。

遇到一些不等值查询，只能试用NL连接。

当然最好：驱动表的限制条件所在的列有索引，被驱动表的连接条件所在的列也有索引。

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