我有一个带有索引“create_at”字段的表。我需要做一些聚合查询，但我不明白使用索引来减少范围是无效的。
所有表格：
SELECT COUNT(1) FROM subscriptions
=＆GT; 412288行
仅2011年：
SELECT COUNT(1) FROM subscriptions
WHERE (created_at BETWEEN '2011-01-01 00:00:00' AND '2011-12-31 23:59:59')
=＆GT; 95217行
按月计算2011年总数：
SELECT SQL_NO_CACHE MONTH(created_at) AS agg_created_at, SUM(amount) AS agg_amount
FROM subscriptions
WHERE (created_at BETWEEN '2011-01-01 00:00:00' AND '2011-12-31 23:59:59')
GROUP BY agg_created_at
ORDER BY agg_created_at
=＆GT; 0.1398秒
+----+-------------+---------------+------+---------------+------+---------+------+--------+----------------------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+---------------+------+---------------+------+---------+------+--------+----------------------------------------------+
| 1 | SIMPLE | subscriptions | ALL | created_at | NULL | NULL | NULL | 422807 | Using where; Using temporary; Using filesort |
+----+-------------+---------------+------+---------------+------+---------+------+--------+----------------------------------------------+
与强制索引相同的查询：
SELECT SQL_NO_CACHE MONTH(created_at) AS agg_created_at, SUM(amount) AS agg_amount
FROM subscriptions FORCE INDEX(created_at)
WHERE (created_at BETWEEN '2011-01-01 00:00:00' AND '2011-12-31 23:59:59')
GROUP BY agg_created_at
ORDER BY agg_created_at
=＆GT; 0.1283秒
+----+-------------+---------------+-------+---------------+------------+---------+------+--------+----------------------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+---------------+-------+---------------+------------+---------+------+--------+----------------------------------------------+
| 1 | SIMPLE | subscriptions | range | created_at | created_at | 8 | NULL | 194106 | Using where; Using temporary; Using filesort |
+----+-------------+---------------+-------+---------------+------------+---------+------+--------+----------------------------------------------+
优化器在这里看起来很聪明，因为使用索引来缩小范围是无效的，但我不明白为什么。如果有人有解释？