MySQL Explain具有与慢查询日志不同的行数

Question

我在慢查询日志中有这个条目：

# User@Host: user[host] @  [ip]
# Thread_id: 1514428  Schema: db  Last_errno: 0  Killed: 0
# Query_time: 2.795454  Lock_time: 0.000116  Rows_sent: 15  Rows_examined: 65207  Rows_affected: 0  Rows_read: 65207
# Bytsent: 26618
SET timestamp=1407511874;
select off.*,translated_title,translated_description
from products off  USE INDEX(id_viewed)
INNER JOIN members mem ON off.uid = mem.id
Left Join product_language_new pol ON off.id = pol.offer_id and pol.language='en'
where off.approved=1
order by off.viewed
LIMIT 15; 

当我解释这个问题时，它绝对没问题。

mysql> explain select off.*,translated_title,translated_description
from products off  USE INDEX(id_viewed)
INNER JOIN members mem ON off.uid = mem.id
Left Join product_language_new pol ON off.id = pol.offer_id and pol.language='en'
where off.approved=1
order by off.viewed
LIMIT 15;

+----+-------------+-------+--------+-------------------------+-------------+---------+---------------------------+------+-------------+
| id | select_type | table | type   | possible_keys           | key         | key_len | ref                       | rows | Extra       |
+----+-------------+-------+--------+-------------------------+-------------+---------+---------------------------+------+-------------+
|  1 | SIMPLE      | off   | index  | NULL                    | id_viewed   | 4       | NULL                      |    3 | Using where |
|  1 | SIMPLE      | mem   | eq_ref | PRIMARY                 | PRIMARY     | 4       | db.off.uid                |    1 | Using index |
|  1 | SIMPLE      | pol   | ref    | offer_id,id_language    | offer_id    | 5       | db.off.id                 |    4 |             |
+----+-------------+-------+--------+-------------------------+-------------+---------+---------------------------+------+-------------+
3 rows in set (0.17 sec)

如何优化此查询？为什么解释显示3行，慢查询日志说它检查了65207行。

是的，我试过FORCE INDEX，没有强制索引，什么不强，它只会变得更糟。

Answer 1

该查询似乎无法使用id_viewed，因为它不在possible_keys列中，extra列只是Using where。

我也猜测rows指的是你有LIMIT ed的返回行，而不是它必须扫描的行数..你可以删除LIMIT EXPLAIN。

我会在(id, approved)上尝试一个复合索引，也可能在(viewed)上尝试一个合并索引。