MYSQL 优化器预处理阶段对子查询的优化

 

最近看了下MYSQL优化器预处理阶段对子查询的优化, 同时还有些疑问发出来向大家请教下。

sql_resolver.cc

 

JOIN::prepare() 预处理阶段

1) remove_redundant_subquery_clauses()方法 去除子查询中冗余语句

  MySQL 中的子查询类型:enum subs_type {UNKNOWN_SUBS, SINGLEROW_SUBS, EXISTS_SUBS, IN_SUBS, ALL_SUBS, ANY_SUBS};

  MySQL 会对EXISTS_SUBS, IN_SUBS, ALL_SUBS, ANY_SUBS 这四种类型的子查询过滤冗余语句,规则如下:

  a) 如果有order by 直接去掉, 因为这里排序是没有意义的 例如:

   select * from t1 where name in ( select name from t2 where t2.col = xxx order by name) -->

   select * from t1 where name in ( select name from t2 where t2.col = xxx ) 

  b) 如果有distinct 直接去掉,其实这里有个疑问:

    如果子查询有100W条记录,去重后只剩下100条记录了这样比较100条记录,否则 id > all 的话需要比较100w条记录呢,这是否说明distinct还是有效果的呢?

    例如:

   select * from t1 where id > all ( select distinct(name) from t2 where t2.col = xxx ) -->

   select * from t1 where id > all ( select name from t2 where t2.col = xxx ) 

  c) 如果子查询有group by且不包含聚合函数和having的条件 直接去掉。

    为何要加上两个非条件呢?

    聚合函数说明需要输出统计信息所以group by是有用的,having使用过滤分组的同样这样的group by也是有用的,所以这这种情况的group by需要保留。

   例如:

   select * from t1 where id in ( select id from t2  where t2.col = xxx group by t2.name) -- >

   select * from t1 where id in ( select id from t2  where t2.col = xxx )

 

2) resovle_subquery()

  a)转化子查询到半连接。

  优点:子查询中全表扫描外层表的每条记录和子查询做join(5.6之前),半连接(上拉/扁平化)将子查询的表上拉到外面做join操作

  这样可以充分使用join的优化策略。

    注意 下面的这个例子还有个对外表去重操作,至于如何去重取决于优化器的半连接优化策略。

     1. 重复剔除:join结果到临时表但是在join列上加上主键,这样临时表中外层表就不会有重复的记录了  

     2. 松散扫描:子查询结果有序且子查询的join列有索引,则对子查询做分组且join每组的第一条记录

     3. 第一次匹配:外层表join到子查询的第一条符合记录就退出这条记录的join操作,接着做外层表的下一条记录join子查询

     4. 索引式物化:子查询物化到临时表,(MySQL自动为临时表创建索引),可以把临时表当成内表利用临时的索引来做join。

     5. 扫描式物化:子查询物化到临时表,但是子查询小结果集的临时表被当成了外表,所以做全表扫描。

  例如:

   select t1.* from t1 where t1.xxx in ( select t2.xxx from t2 )  -->

   select t1.* from t1 join t2 on t1.xxx = t2.xxx

  前置条件:

     /*

        Check if we're in subquery that is a candidate for flattening into a

        semi-join (which is done in flatten_subqueries()). The requirements are:

          1. Subquery predicate is an IN/=ANY subquery predicate

          2. Subquery is a single SELECT (not a UNION)

          3. Subquery does not have GROUP BY

          4. Subquery does not use aggregate functions or HAVING

          5. Subquery predicate is at the AND-top-level of ON/WHERE clause

            a) select xxx from t1 where t1.xxx in ()

            b) select t1.xx from t1 join t2 on  t2.xx2 in ( )

          6. We are not in a subquery of a single table UPDATE/DELETE that 

               doesn't have a JOIN (TODO: We should handle this at some

               point by switching to multi-table UPDATE/DELETE)

               rm 单表的UPDATE/DELETE的子查询是不行,因为UPDATE/DELETE没有join一说

          7. We're not in a confluent table-less subquery, like "SELECT 1".

              rm  无表子句不处理

          8. No execution method was already chosen (by a prepared statement)

          9. Parent select is not a confluent table-less select

             rm  父表不能是无表查询 如 select 1

          10. Neither parent nor child select have STRAIGHT_JOIN option.

      */

 

 b)物化标识子查询

   将非相关子查询保存到临时表中或者缓存中。

 c) 执行in 向 exists转换(优点:这样exists就可以使用半连接的优化策略了)

   例子:

    outer_expr IN (SELECT inner_expr FROM … WHERE subquery_where) -->

    EXISTS (SELECT 1 FROM … WHERE subquery_where AND outer_expr=inner_expr)

   但是还有两种情况需要考虑outer_expr为null的情况和inner_expr为null的情况,这两点也是我比较疑惑的地方

   例如,有如下表a1:

   

    我执行select * from a1 where id in ( select df from a1)  -->无记录 

    select * from a1 as t1 where exists ( select 1 from a1 t2 where (t1.id = t2.df or t2.df is null ) )  --> 三条记录

    结果竟然不一样!这是为什么呢?

 d)<op> ALL/ANY/SOME 向MIN MAX转换 OP为大于或者小于操作,例子会将子查询转化成下面两种其中的一种。

   例子:(oe 外表的表达式, ie 内表的表达式, cmp为 ">  <") 

    oe $cmp$ (SELECT ie FROM ... WHERE subq_where ... HAVING subq_having) --> 

    oe $cmp$ (SELECT MAX(...) )  // handled by Item_singlerow_subselect

    oe $cmp$ <max>(SELECT ...)   // handled by Item_maxmin_subselect

   前置条件:

      /*

        If this is an ALL/ANY single-value subquery predicate, try to rewrite

        it with a MIN/MAX subquery.

    

        E.g. SELECT * FROM t1 WHERE b > ANY (SELECT a FROM t2) can be rewritten

        with SELECT * FROM t1 WHERE b > (SELECT MIN(a) FROM t2).

    

        A predicate may be transformed to use a MIN/MAX subquery if it:

        1. has a greater than/less than comparison operator, and

        2. is not correlated with the outer query, and

            rm  非相关子查询(与外表无关) select t1.xx from t1 where t1.xxx > any ( select xxx from t2 where t2.xx = 123 )

        3. UNKNOWN results are treated as FALSE, or can never be generated, and

            rm 未知结果被当成false处理,或者永远不会生成未知结果

      */

 e) 使用值替代标量子查询

  (标量子查询:select (subquery) from table)其中subquery只能为单列和单值。

 

 

 

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