MySQL中的semi-join

所谓的semi-join是指semi-join子查询。 当一张表在另一张表找到匹配的记录之后，半连接（semi-jion）返回第一张表中的记录。与条

1. 背景介绍
什么是semi-join？
所谓的semi-join是指semi-join子查询。 当一张表在另一张表找到匹配的记录之后，半连接（semi-jion）返回第一张表中的记录。与条件连接相反，即使在右节点中找到几条匹配的记录，左节点 的表也只会返回一条记录。另外，右节点的表一条记录也不会返回。半连接通常使用IN 或 EXISTS 作为连接条件。 该子查询具有如下结构：

SELECT ... FROM outer_tables WHERE expr IN (SELECT ... FROM inner_tables ...) AND ...

即在where条件的“IN”中的那个子查询。
这种查询的特点是我们只关心outer_table中与semi-join相匹配的记录。
换句话说，最后的结果集是在outer_tables中的，，而semi-join的作用只是对outer_tables中的记录进行筛选。这也是我们进行 semi-join优化的基础，即我们只需要从semi-join中获取到最少量的足以对outer_tables记录进行筛选的信息就足够了。
所谓的最少量，体现到优化策略上就是如何去重。
以如下语句为例：

Country where Country.Code in (select City.country from City );

当中的semi-join： “

select City.country from City

” 可能返回的结果集如下： China(Beijin), China(Shanghai), France(Paris)...
我们可以看到这里有2个China，分别来至2条城市记录Beijin和Shanghai, 但实际上我们只需要1个China就足够对outer_table

Country进行筛选了。所以我们需要去重。

2. Mysql支持的Semi-join策略
Mysql支持的semi-join策略主要有5个，它们分别为：
1. DuplicateWeedout: 使用临时表对semi-join产生的结果集去重。

Duplicate Weedout: Run the semi-join as if it was a join and remove duplicate records using a temporary table.

对应的匹配条件为：

2. FirstMatch: 只选用内部表的第1条与外表匹配的记录。

FirstMatch: When scanning the inner tables for row combinations and there are multiple instances of a given value group, choose one rather than returning them all. This "shortcuts" scanning and eliminates production of unnecessary rows.

对应的匹配条件为：

3. LooseScan: 把inner-table数据基于索引进行分组，取每组第一条数据进行匹配。

LooseScan: Scan a subquery table using an index that enables a single value to be chosen from each subquery's value group.

对应的匹配条件为：

4. Materializelookup: 将inner-table去重固化成临时表，遍历outer-table，然后在固化表上去寻找匹配。

Materialize the subquery into a temporary table with an index and use the temporary table to perform a join. The index is used to remove duplicates. The index might also be used later for lookups when joining the temporary table with the outer tables; if not, the table is scanned.
对应的匹配条件：

5. MaterializeScan: 将inner-table去重固化成临时表，遍历固化表，然后在outer-table上寻找匹配。

对应的条件：

Each of these strategies except Duplicate Weedout can be enabled or disabled using the system variable. The semijoin flag controls whether semi-joins are used. If it is set to on, the firstmatch, loosescan, and materialization flags enable finer control over the permitted semi-join strategies. These flags are on by default.

The use of semi-join strategies is indicated in EXPLAIN output as follows:

Semi-joined tables show up in the outer select. EXPLAIN EXTENDED plus SHOW WARNINGS shows the rewritten query, which displays the semi-join structure. From this you can get an idea about which tables were pulled out of the semi-join. If a subquery was converted to a semi-join, you will see that the subquery predicate is gone and its tables and WHERE clause were merged into the outer query join list and WHERE clause.

Temporary table use for Duplicate Weedout is indicated by Start temporary and End temporary in the Extra column. Tables that were not pulled out and are in the range of EXPLAIN output rows covered by Start temporary and End temporary will have their rowid in the temporary table.

FirstMatch(tbl_name) in the Extra column indicates join shortcutting.

LooseScan(m..n) in the Extra column indicates use of the LooseScan strategy. m and n are key part numbers.

As of MySQL 5.6.7, temporary table use for materialization is indicated by rows with a select_type value of MATERIALIZED and rows with a table value of .