mysql的delete没走索引？

表做了分析和优化：

ANALYZE TABLE Currsession_table;
OPTIMIZE TABLE Currsession_table;

两种写法严重的性能差异：

原因rcdate字段类型为datetime


下面的写法多了.0000毫秒信息，导致类型不同比较没有走索引。
select貌似走了索引，但rows显示还是全表的数据180多万，明显不正常。
delete完全没有走索引。
更换写法后可以看到，如果正常走索引应该就1条记录。

root@192.168.1.51 [jyc] >explain select count(*) FROM Currsession_table FORCE INDEX(idx_recdate) WHERE recdate < DATE_ADD(SYSDATE(), INTERVAL - (1500/86400) * 86400 SECOND);
+----+-------------+-------------------+------------+-------+---------------+-------------+---------+------+---------+----------+--------------------------+
| id | select_type | table             | partitions | type  | possible_keys | key         | key_len | ref  | rows    | filtered | Extra                    |
+----+-------------+-------------------+------------+-------+---------------+-------------+---------+------+---------+----------+--------------------------+
|  1 | SIMPLE      | Currsession_table | NULL       | index | NULL          | idx_recdate | 6       | NULL | 1823021 |    33.33 | Using where; Using index |
+----+-------------+-------------------+------------+-------+---------------+-------------+---------+------+---------+----------+--------------------------+
1 row in set, 1 warning (0.01 sec)

root@192.168.1.51 [jyc] >explain select count(*) FROM Currsession_table use INDEX(idx_recdate) WHERE recdate < DATE_ADD(SYSDATE(), INTERVAL - (1500/86400) * 86400 SECOND);
+----+-------------+-------------------+------------+-------+---------------+-------------+---------+------+---------+----------+--------------------------+
| id | select_type | table             | partitions | type  | possible_keys | key         | key_len | ref  | rows    | filtered | Extra                    |
+----+-------------+-------------------+------------+-------+---------------+-------------+---------+------+---------+----------+--------------------------+
|  1 | SIMPLE      | Currsession_table | NULL       | index | NULL          | idx_recdate | 6       | NULL | 1776171 |    33.33 | Using where; Using index |
+----+-------------+-------------------+------------+-------+---------------+-------------+---------+------+---------+----------+--------------------------+
1 row in set, 1 warning (0.00 sec)

root@192.168.1.51 [jyc] >explain delete FROM Currsession_table use INDEX(idx_recdate) WHERE recdate < DATE_ADD(SYSDATE(), INTERVAL - (1500/86400) * 86400 SECOND);
ERROR 1064 (42000): You have an error in your SQL syntax; check the manual that corresponds to your MySQL server version for the right syntax to use near 'use INDEX(idx_recdate) WHERE recdate < DATE_ADD(SYSDATE(), INTERVAL - (1500/8640' at line 1
root@192.168.1.51 [jyc] >explain delete FROM Currsession_table WHERE recdate < DATE_ADD(SYSDATE(), INTERVAL - (1500/86400) * 86400 SECOND);
+----+-------------+-------------------+------------+------+---------------+------+---------+------+---------+----------+-------------+
| id | select_type | table             | partitions | type | possible_keys | key  | key_len | ref  | rows    | filtered | Extra       |
+----+-------------+-------------------+------------+------+---------------+------+---------+------+---------+----------+-------------+
|  1 | DELETE      | Currsession_table | NULL       | ALL  | NULL          | NULL | NULL    | NULL | 1773948 |   100.00 | Using where |
+----+-------------+-------------------+------------+------+---------------+------+---------+------+---------+----------+-------------+
1 row in set, 1 warning (0.00 sec)

root@192.168.1.51 [jyc] >select count(*) FROM Currsession_table use INDEX(idx_recdate) WHERE recdate < DATE_ADD(SYSDATE(), INTERVAL - (1500/86400) * 86400 SECOND);
+----------+
| count(*) |
+----------+
|        0 |
+----------+
1 row in set (6.71 sec)

root@192.168.1.51 [jyc] >select count(*) FROM Currsession_table WHERE recdate < DATE_ADD(SYSDATE(), INTERVAL - (1500/86400) * 86400 SECOND);
+----------+
| count(*) |
+----------+
|        0 |
+----------+
1 row in set (6.86 sec)

root@192.168.1.51 [jyc] >explain select count(*) FROM Currsession_table use INDEX(idx_recdate) WHERE recdate < DATE_ADD(SYSDATE(), INTERVAL - (1500/86400) * 86400 SECOND);
+----+-------------+-------------------+------------+-------+---------------+-------------+---------+------+---------+----------+--------------------------+
| id | select_type | table             | partitions | type  | possible_keys | key         | key_len | ref  | rows    | filtered | Extra                    |
+----+-------------+-------------------+------------+-------+---------------+-------------+---------+------+---------+----------+--------------------------+
|  1 | SIMPLE      | Currsession_table | NULL       | index | NULL          | idx_recdate | 6       | NULL | 1787580 |    33.33 | Using where; Using index |
+----+-------------+-------------------+------------+-------+---------------+-------------+---------+------+---------+----------+--------------------------+
1 row in set, 1 warning (0.00 sec)

root@192.168.1.51 [jyc] >explain delete  FROM Currsession_table WHERE recdate <now() -interval 25 minute;
+----+-------------+-------------------+------------+-------+---------------+-------------+---------+-------+------+----------+-------------+
| id | select_type | table             | partitions | type  | possible_keys | key         | key_len | ref   | rows | filtered | Extra       |
+----+-------------+-------------------+------------+-------+---------------+-------------+---------+-------+------+----------+-------------+
|  1 | DELETE      | Currsession_table | NULL       | range | idx_recdate   | idx_recdate | 6       | const |    1 |   100.00 | Using where |
+----+-------------+-------------------+------------+-------+---------------+-------------+---------+-------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)

root@192.168.1.51 [jyc] >explain select count(*)  FROM Currsession_table WHERE recdate <now() -interval 25 minute;
+----+-------------+-------------------+------------+-------+---------------+-------------+---------+------+------+----------+--------------------------+
| id | select_type | table             | partitions | type  | possible_keys | key         | key_len | ref  | rows | filtered | Extra                    |
+----+-------------+-------------------+------------+-------+---------------+-------------+---------+------+------+----------+--------------------------+
|  1 | SIMPLE      | Currsession_table | NULL       | range | idx_recdate   | idx_recdate | 6       | NULL |    1 |   100.00 | Using where; Using index |
+----+-------------+-------------------+------------+-------+---------------+-------------+---------+------+------+----------+--------------------------+
1 row in set, 1 warning (0.00 sec)