MySQL firstmatch strategy

在探讨subquery如 x IN (SELECT XX FROM TABLE)这样的形式的MATCH策略时，不是很清楚实现过程。在网上搜了一下，

地址：http://stackoverflow.com/questions/22300291/what-does-firstmatch-mean-in-a-explain-statement

It is a strategy that avoids the production of duplicates by short-cutting execution as soon as the first genuine match is found.

So FirstMatch(phonenumbers) means that as soon as we have produced one matching record combination, it will short-cut the execution and jump back to the phonenumbers table.

From FirstMatch strategy

• The FirstMatch strategy works by executing the subquery and short-cutting its execution as soon as the first match is found.
• This means, subquery tables must be after all of the parent select‘s tables that are referred from the subquery predicate.
• EXPLAIN shows FirstMatch as "FirstMatch(tableN)".
• The strategy can handle correlated subqueries.
• But it cannot be applied if the subquery has meaningful GROUP BY and/or aggregate functions.
• Use of the FirstMatch strategy is controlled with the firstmatch=on|off flag in the optimizer_switch variable.

MySQL firstmatch strategy

原文：http://www.cnblogs.com/oDoraemon/p/6269168.html