我尝试了几种不同的方法，但我无法弄清楚为什么我的查询不会使用索引。
EXPLAIN SELECT
allocationdate,
SUM(linepersonal) as recpts
FROM
c1.recpt #force index (rbadateptype) #force index (rbADate)
WHERE
allocationdate BETWEEN 20120101 AND 20140630
AND ptype = 3
GROUP BY YEAR(allocationdate);
不强制索引：
+----+-------------+--------+------+----------------------+------+---------+------+----------+----------------------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | extra |
+----+-------------+--------+------+----------------------+------+---------+------+----------+----------------------------------------------+
| 1 | SIMPLE | recptb | ALL | rbADate,rbADatePtype | NULL | NULL | NULL | 23186904 | Using where; Using temporary; Using filesort |
+----+-------------+--------+------+----------------------+------+---------+------+----------+----------------------------------------------+
强制使用rbadateptype：
+----+-------------+--------+------+----------------------+----------------+---------+------+----------+----------------------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | extra |
+----+-------------+--------+------+----------------------+----------------+---------+------+----------+----------------------------------------------+
| 1 | SIMPLE | recptb | ALL | rbADate,rbADatePtype | rbAdatePtype | 9 | NULL | 8199594| Using where; Using temporary; Using filesort |
+----+-------------+--------+------+----------------------+----------------+---------+------+----------+----------------------------------------------+
`allocationDate` date DEFAULT NULL,
`pType` int(10) DEFAULT NULL,
KEY `rbADate` (`allocationDate`),
KEY `rbAdatePtype` (`allocationDate`,`patType`),
我无法解释的一件有趣的事情是，日期的索引如何表示它需要查看7,784,810条记录，但日期和ptype上的综合索引需要查看8,199,594。
我刚注意到的一个更有趣的事情是，使用rbAdate索引时，它表示当select count（1）返回21,610,791时需要查看23,186,904条记录。