为什么要读Query Plan, 在线文档中有句话是这么说的:
While the SQL compiler tries to make the most efficient use of data as specified by the query, sometimes the author of the query knows more about some aspect of the stored data than is evident to the compiler. In this case, the author can make use of the query plan to modify the original query to provide more information or more guidance to the query compiler.
翻译一下是这样:系统给你的查询计划并不总是最好的,如果您能对查询计划,可以人工做更精细的优化。