Home Database Mysql Tutorial 哈希连接(hash join) 原理

哈希连接(hash join) 原理

Jun 07, 2016 pm 05:32 PM

这三类表连接方式是Oracle最基本的连接方式:嵌套循环连接(nested loops join)原理 排序合并连接(sort merge join)的原理 哈希连

这三类表连接方式是Oracle最基本的连接方式:
嵌套循环连接(nested loops join)原理
排序合并连接(sort merge join)的原理

哈希连接(hashjoin)

访问次数:驱动表和被驱动表都只会访问0次或1次。
驱动表是否有顺序:有。
是否要排序:否。
应用场景: 1. 一个大表,一个小表的关联;
2. 表上没有索引;
3. 返回结果集比较大。

原理我们说的简单一点,先把驱动表的关联字段hash到PGA中(当然rowid也在PGA中),然后扫描被驱动表,,取第一条数据,将关联的字段hash 一下探测PGA中的小表,如果匹配则关联,再取第二条........。

下面我们来做个试验:

SQL> create table test1 as select * from dba_objects where rownum SQL> create table test2 as select * from dba_objects where rownum SQL> exec dbms_stats.gather_table_stats(user,'test1');
SQL> exec dbms_stats.gather_table_stats(user,'test2');
SQL> alter session set statistics_level=all;

SQL> select /*+leading(t1) use_hash(t2)*/count(*)
from test1 t1, test2 t2
where t1.object_id = t2.object_id;
COUNT(*)
----------
100

SQL> select * from table(dbms_xplan.display_cursor(null,null,'allstats last'));
PLAN_TABLE_OUTPUT
-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------
SQL_ID 3f2mts0kt82u2, child number 0
-------------------------------------
select /*+leading(t1) use_hash(t2)*/count(*) from test1 t1, test2 t2 where t1.object_id = t2.object_id
Plan hash value: 2544416891

----解释一下:

Starts为该sql执行的次数。
E-Rows为执行计划预计的行数。
A-Rows为实际返回的行数。A-Rows跟E-Rows做比较,就可以确定哪一步执行计划出了问题。
A-Time为每一步实际执行的时间(HH:MM:SS.FF),根据这一行可以知道该sql耗时在了哪个地方。
Buffers为每一步实际执行的逻辑读或一致性读。
Reads为物理读。
OMem、1Mem为执行所需的内存评估值,0Mem为最优执行模式所需内存的评估值,1Mem为one-pass模式所需内存的评估值。
0/1/M 为最优/one-pass/multipass执行的次数。
Used-Mem耗的内存

------------------------------------------------------------------------------------------------------------------
| Id | Operation | Name | Starts | E-Rows | A-Rows | A-Time | Buffers | OMem | 1Mem | Used-Mem |
------------------------------------------------------------------------------------------------------------------
| 1 | SORT AGGREGATE | | 1 | 1 | 1 |00:00:00.01 | 19 | | | |
|* 2 | HASH JOIN | | 1 | 100 | 100 |00:00:00.01 | 19 | 1066K| 1066K| 1162K (0)|
| 3 | TABLE ACCESS FULL| TEST1 | 1| 100 | 100 |00:00:00.01 | 4 | | | |
| 4 | TABLE ACCESS FULL| TEST2 | 1 | 1000 | 1000 |00:00:00.01 | 15 | | | |
------------------------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
2 - access("T1"."OBJECT_ID"="T2"."OBJECT_ID")


SQL> select /*+leading(t1) use_hash (t2)*/count(*)
from test1 t1, test2 t2
where t1.object_id = t2.object_id
and t1.object_id = 99999;
COUNT(*)
----------
0

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