ColumnMeaningidThe SELECT identifierselect_typeThe SELECT typetableThe table for the output rowpartitionsThe matching partitionstypeThe join typepossible_keysThe possible indexes to choosekeyThe index actually chosenkey_lenThe length of the chosen keyrefThe columns compared to the indexrowsEstimate of rows to be examinedfilteredPercentage of rows filtered by table conditionextraAdditional informationid
主要用来分辨查询的类型，是普通查询还是联合查询还是子查询
select_type ValueMeaning解释SIMPLESimple SELECT (not using UNION or subqueries)SIMPLE(简单SELECT,不使用UNION或子查询等)PRIMARYOutermost SELECT查询中若包含任何复杂的子部分,最外层的select被标记为PRIMARY)UNIONSecond or later SELECT statement in a UNIONUNION中的第二个或后面的SELECT语句)DEPENDENT UNIONSecond or later SELECT statement in a UNION, dependent on outer queryUNION中的第二个或后面的SELECT语句，取决于外面的查询UNION RESULTResult of a UNION.UNION的结果SUBQUERYFirst SELECT in subquery子查询中的第一个SELECTDEPENDENT SUBQUERYFirst SELECT in subquery, dependent on outer query子查询中的第一个SELECT，取决于外面的查询DERIVEDDerived table派生表的SELECT, FROM子句的子查询UNCACHEABLE SUBQUERYA subquery for which the result cannot be cached and must be re-evaluated for each row of the outer query一个子查询的结果不能被缓存，必须重新评估外链接的第一行UNCACHEABLE UNIONThe second or later select in a UNION that belongs to an uncacheable subquery (see UNCACHEABLE SUBQUERY)
