Mysql无法使用范围查询列之后的其他索引列（以及5.6版本的ICP）

mysql5.6版本之前没有加入index condition pushdown，所以索引逻辑还是这样的：

即便对于复合索引，从第一列开始先确定第一列索引范围，如果范围带=号，则对于=号情况，确定第二列索引范围加入索引结果集里，每列的处理方式都是一样的。

确定完索引范围后，则回表查询数据，再用剩下的where条件进行过滤判断。

mysql5.6后加入了ICP，对于确定完了索引范围后，会用剩下的where条件对索引范围再进行一次过滤，然后再回表，再用剩下的where条件进行过滤判断。（减少回表记录数量）。

The optimizer attempts to use additional key parts to determine the interval as long as the comparison operator is =, <=>, or IS NULL. If the operator is >, <, >=, <=, !=, <>, BETWEEN, or LIKE, the optimizer uses it but considers no more key parts.For the following expression, the optimizer uses = from the first comparison. It also uses >= from the second comparison but considers no further key parts and does not use the third comparison for interval construction: key_part1 = 'foo' AND key_part2 >= 10 AND key_part3 > 10 The single interval is: ('foo',10,-inf) < (key_part1,key_part2,key_part3) < ('foo',+inf,+inf)http://dev.mysql.com/doc/refman/5.5/en/range-optimization.html#range-access-multi-part