MySQL有一个内部查询优化器，在大多数情况下处理这些事情。所以，通常，你不需要担心它。

MySQL has an internal query optimizer that takes care of such things in most cases. So, typically, you don't need to worry about it.

但是，当然，查询优化器不是万无一失的。所以...

But, of course, the query optimizer is not foolproof. So...

很抱歉，您需要熟悉 EXPLAIN if您怀疑查询的运行效率可能低于其应该的效率。

Sorry to do this to you, but you'll want to get familiar with EXPLAIN if you suspect that a query may be running less efficiently than it should.

http://dev.mysql.com/doc/refman/5.0/en/explain.html