我在2个表上有匹配的Btree索引,但解释计划说引擎正在对其中一个表进行全面扫描,而且速度非常慢。部署时,两者都以〜750,000行开始,关系是em_localitems:em_submenuitemassoc => 1:N。我的理解是下面的**指数应该在下面的查询中运行良好,因为它们是从左到右使用的。
CREATE TABLE IF NOT EXISTS em_localitems (
localitemid int(11) NOT NULL AUTO_INCREMENT,
profitcenterid int(11) DEFAULT NULL,
productid int(11) DEFAULT NULL,
PRIMARY KEY (localitemid),
UNIQUE KEY locationid (profitcenterid,productid),
**KEY productid_2 (productid,profitcenterid)**
) ENGINE=MyISAM DEFAULT CHARSET=latin1 ;
CREATE TABLE IF NOT EXISTS em_submenuitemassoc (
submenuitemassocid int(11) NOT NULL AUTO_INCREMENT,
productid int(11) NOT NULL,
profitcenterid int(11) NOT NULL DEFAULT '0',
submenuid int(11) NOT NULL,
enddate datetime DEFAULT NULL,
PRIMARY KEY (submenuitemassocid),
UNIQUE KEY productid (productid,profitcenterid,submenuid),
**KEY productid_3 (productid,profitcenterid,submenuid)**
) ENGINE=MyISAM DEFAULT CHARSET=latin1;
这是查询:
SELECT * from
em_submenuitemassoc sm
LEFT outer JOIN em_localitems li2 on li2.productid=sm.productid
and li2.profitcenterid=sm.profitcenterid
and sm.profitcenterid is not null
我也试过索引提示:
SELECT * from
em_submenuitemassoc sm **use index(productid_3)**
LEFT outer JOIN em_localitems li2 on li2.productid=sm.productid
and li2.profitcenterid=sm.profitcenterid
and sm.profitcenterid is not null
“显示来自em_submenuitemassoc的索引;”返回:
+---------------------+------------+----------------+--------------+--------------------+-----------+-------------+----------+--------+------+------------+---------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment |
+---------------------+------------+----------------+--------------+--------------------+-----------+-------------+----------+--------+------+------------+---------+
| em_submenuitemassoc | 1 | productid_3 | 1 | productid | A | 1136 | NULL | NULL | | BTREE | |
| em_submenuitemassoc | 1 | productid_3 | 2 | profitcenterid | A | 1136 | NULL | NULL | | BTREE | |
| em_submenuitemassoc | 1 | productid_3 | 3 | submenuid | A | 1136 | NULL | NULL | | BTREE | |
+---------------------+------------+----------------+--------------+--------------------+-----------+-------------+----------+--------+------+------------+---------+
这是解释计划:
+----+-------------+-------+------+------------------------+-------------+---------+--------------------------------------------------+------+-------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-------+------+------------------------+-------------+---------+--------------------------------------------------+------+-------+
| 1 | SIMPLE | sm | ALL | NULL | NULL | NULL | NULL | 1136 | |
| 1 | SIMPLE | li2 | ref | locationid,productid_2 | productid_2 | 10 | datatest.sm.profitcenterid,datatest.sm.productid | 1 | |
+----+-------------+-------+------+------------------------+-------------+---------+--------------------------------------------------+------+-------+
这有什么可怕的错误?
答案 0 :(得分:2)
查看表格 sm 的查询附件type
是否为“全部”。无论连接条件如何,优化器都知道它必须读取 sm 的所有行。
这可能是因为您正在使用LEFT OUTER JOIN
返回所有连接中左表的行,而不管连接的右表中是否存在匹配的行
您还在使用SELECT *
,因此无论如何都必须获取 sm 的所有列。如果查询只需要索引中的列,则可以跳过读取基表。
因此,使用索引在此查询中没有任何好处,并且MySQL通过跳过阅读索引来获得更高的效率。