32.读书笔记收获不止Oracle之表链接的驱动顺序

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32.读书笔记收获不止Oracle之表链接的驱动顺序

1. 嵌套循环驱动顺序

T1和T2表草考前面笔记。

sql>Alter session set statistics_level=all;

1.1实验1

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.

1.2实验2

sql>select /*+leading(t2) use_nl(t1)*/ * 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 axb2hvwr253j8,child number 0

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

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

and t1.n=19

Plan hash value: 4016936828

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

-----

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

PLAN_TABLE_OUTPUT

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

ers |

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

-----

| 0| SELECT STATEMENT | | 1 | | 1 |00:00:00.93 |

701K|

| 1| NESTED LOOPS | | 1 | 1 | 1 |00:00:00.93 |

701K|

PLAN_TABLE_OUTPUT

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

| 2| TABLE ACCESS FULL| T2 | 1 | 100K| 100K|00:00:00.01 | 1

006 |

|* 3| TABLE ACCESS FULL| T1 |100K| 1 | 1 |00:00:00.89 |

700K|

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

-----

Predicate Information (identified byoperation id):

PLAN_TABLE_OUTPUT

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

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

3- filter(("T1"."ID"="T2"."T1_ID" AND"T1"."N"=19))

21 rows selected.

采用不同的驱动表，使用的BUFFER是不一样的，t1为驱动表的时候是1014，t2为驱动表的话使用了701k大小。

T1作为驱动表访问的情况下，T2表只被访问了1次。而t2 表作为驱动表被访问的情况下，T1表被访问了10000次，因为T1结果返回1条记录，而T2结构返回10000条记录。

嵌套循环连接要注意驱动表的顺序。

2. 哈希连接的表驱动顺序

2.1实验1

sql>select /*+leading(t1) use_hash(t2)*/ * from t1,'allstatslast'));

PLAN_TABLE_OUTPUT

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

sql_ID 0x20q3zf8pn79,child number 0

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

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

and t1.n=19

Plan hash value: 1838229974

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

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

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

PLAN_TABLE_OUTPUT

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

ers |OMem | 1Mem | Used-Mem |

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

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

| 0| SELECT STATEMENT | | 1 | | 1 |00:00:00.05 | 1

013 | | | |

|* 1| HASH JOIN | | 1 | 1 | 1 |00:00:00.05 | 1

013 | 960K| 960K|403K (0)|

PLAN_TABLE_OUTPUT

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

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

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)

22 rows selected.

2.2实验2

sql>select /*+leading(t2) use_hash(t1)*/* from t1,t2 where t1.id=t2.t1_id and t1.n=19;

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

PLAN_TABLE_OUTPUT

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

sql_ID 1m1vnuurcxcm3,child number 0

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

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

and t1.n=19

Plan hash value: 2959412835

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

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

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

PLAN_TABLE_OUTPUT

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

ers |OMem | 1Mem | Used-Mem |

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

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

| 0| SELECT STATEMENT | | 1 | | 1 |00:00:00.04 | 1

013 | | | |

|* 1| HASH JOIN | | 1 | 1 | 1 |00:00:00.04 | 1

013 | 11M| 2469K| 12M (0)|

PLAN_TABLE_OUTPUT

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

| 2| TABLE ACCESS FULL| T2 | 1 | 100K| 100K|00:00:00.01 | 1

005 | | | |

|* 3| TABLE ACCESS FULL| T1 | 1 | 1 | 1 |00:00:00.01 |

8 | | | |

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

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

Predicate Information (identified byoperation id):

PLAN_TABLE_OUTPUT

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

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

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

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

22 rows selected.

使用的BUFFER是一致的，Used-Mem 实验1是403k,实验2是12M。

在哈希连接中驱动表的顺序也是非常重要的。

3. 排序合拼连接的表驱动顺序

3.1实验1

sql>select /*+leading(t1) use_merge(t2)*/ * from t1,'allstats last'));

PLAN_TABLE_OUTPUT

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

sql_ID afuusm6140307,child number 0

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

select /*+leading(t1) use_merge(t2)*/ *from t1,t2 where t1.id=t2.t1_id

and t1.n=19

Plan hash value: 412793182

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

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

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

PLAN_TABLE_OUTPUT

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

fers | OMem| 1Mem | Used-Mem |

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

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

| 0| SELECT STATEMENT | | 1 | | 1 |00:00:00.07 |

1012 | | | |

| 1| MERGE JOIN | | 1 | 1 | 1 |00:00:00.07 |

1012 | | | |

PLAN_TABLE_OUTPUT

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

| 2| SORT JOIN | | 1 | 1 | 1 |00:00:00.01 |

7| 2048 | 2048 | 2048 (0)|

|* 3| TABLE ACCESS FULL| T1 | 1 | 1 | 1 |00:00:00.01 |

7| | | |

|* 4| SORT JOIN | | 1 | 100K| 1|00:00:00.07 |

1005 | 9762K| 1209K| 8677K (0)|

| 5| TABLE ACCESS FULL| T2 | 1 | 100K| 100K|00:00:00.02 |

1005 | | | |

PLAN_TABLE_OUTPUT

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

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

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

Predicate Information (identified byoperation id):

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

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

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

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

PLAN_TABLE_OUTPUT

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

25 rows selected.

3.2实验2

sql>select /*+leading(t2) use_merge(t1)*/ * from t1,'allstats last'));

PLAN_TABLE_OUTPUT

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

sql_ID dz2rnd44fg2jb,child number 0

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

select /*+leading(t2) use_merge(t1)*/ *from t1,t2 where t1.id=t2.t1_id

and t1.n=19

Plan hash value: 1792967693

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

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

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

PLAN_TABLE_OUTPUT

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

fers | OMem| 1Mem | Used-Mem |

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

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

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

1012 | | | |

| 1| MERGE JOIN | | 1 | 1 | 1 |00:00:00.06 |

1012 | | | |

PLAN_TABLE_OUTPUT

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

| 2| SORT JOIN | | 1 | 100K| 20|00:00:00.06 |

1005 | 9762K| 1209K| 8677K (0)|

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

1005 | | | |

|* 4| SORT JOIN | | 20 | 1 | 1 |00:00:00.01 |

7| 2048 | 2048 | 2048 (0)|

|* 5| TABLE ACCESS FULL| T1 | 1 | 1 | 1 |00:00:00.01 |

7| | | |

PLAN_TABLE_OUTPUT

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

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

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

Predicate Information (identified byoperation id):

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

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

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

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

PLAN_TABLE_OUTPUT

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

25 rows selected.

排序合并连接实验1和实验2 的效率是一样的，

小结：

嵌套循环连接和哈希连接有驱动顺序，驱动表的顺序不同将影响表连接的性能，而排序合并连接没有驱动的概念，无论哪张表在前都无妨。

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