Mysql在执行时，并不一定就会按照我们写的顺序执行，同时也不需要我们人为的从左到右的安排where后面的条件，mysql优化器会重写sql，如何才能看到mysql优化器重写后的sql呢？此时需要参考explain extended和show warnings这两个命令啦。
explain extended sql语句，然后show warnings查看。explain extended会输出sql的执行计划，查询记录的方式(全表扫描、全索引扫描、索引范围扫描等)、是否用上索引。show warnings会看到优化器重写后的sql。例如：
mysql> EXPLAIN extended SELECT
IFNULL(SUM(min_investment_amt),0.00)
FROM
user_coupon uc,
event_package ep
WHERE
uc.pid = ep.pid
AND ep.show_channel = '1'
AND uc.`status` = 0
AND uc.valid_to <= CURDATE()
AND uc.uid = '75B87DA8BBF3448BBA431ACC511EEAF0';
+----+-------------+-------+------+-------------------------+---------+---------+----------------------+------+----------+------------------------------------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------+-------------------------+---------+---------+----------------------+------+----------+------------------------------------+
| 1 | SIMPLE | uc | ref | uid,pid,status,valid_to | uid | 194 | const | 294 | 100 | Using index condition; Using where |
| 1 | SIMPLE | ep | ref | idx_pid | idx_pid | 98 | trc_coupon_db.uc.pid | 1 | 100 | Using where |
+----+-------------+-------+------+-------------------------+---------+---------+----------------------+------+----------+------------------------------------+
2 rows in set
mysql> show warnings;
+-------+------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Level | Code | Message |
+-------+------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Note | 1003 | /* select#1 */ select ifnull(sum(`trc_coupon_db`.`uc`.`min_investment_amt`),0.00) AS `IFNULL(SUM(min_investment_amt),0.00)` from `trc_coupon_db`.`user_coupon` `uc` join `trc_coupon_db`.`event_package` `ep` where ((`trc_coupon_db`.`uc`.`status` = 0) and (`trc_coupon_db`.`ep`.`pid` = `trc_coupon_db`.`uc`.`pid`) and (`trc_coupon_db`.`ep`.`show_channel` = '1') and (`trc_coupon_db`.`uc`.`valid_to` <= <cache>(curdate())) and (`trc_coupon_db`.`uc`.`uid` = '75B87DA8BBF3448BBA431ACC511EEAF0')) |
+-------+------+-----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set
mysql> SELECT
IFNULL(
SUM(min_investment_amt),
0.00
)
FROM
user_coupon uc,
event_package ep
WHERE
uc.pid = ep.pid
AND ep.show_channel = '1'
AND uc.`status` = 0
AND uc.valid_to <= CURDATE()
AND uc.uid = '75B87DA8BBF3448BBA431ACC511EEAF0';
mysql优化器会将此语句优化成内连接
SELECT
ifnull(
sum(
`trc_coupon_db`.`uc`.`min_investment_amt`
),
0.00
) AS `IFNULL(SUM(min_investment_amt),0.00)`
FROM
`trc_coupon_db`.`user_coupon` `uc`
JOIN `trc_coupon_db`.`event_package` `ep`
WHERE
(
(
`trc_coupon_db`.`uc`.`status` = 0
)
AND (
`trc_coupon_db`.`ep`.`pid` = `trc_coupon_db`.`uc`.`pid`
)
AND (
`trc_coupon_db`.`ep`.`show_channel` = '1'
)
AND (
`trc_coupon_db`.`uc`.`valid_to` <= < CACHE > (curdate())
)
AND (
`trc_coupon_db`.`uc`.`uid` = '75B87DA8BBF3448BBA431ACC511EEAF0'
)
)
