标签：使用介绍 成功 命令 xpl ddr reference cte state 接下来
MySQL 提供了一个 EXPLAIN 命令, 它可以对 SELECT 语句进行分析, 并输出 SELECT 执行的详细信息, 以供开发人员针对性优化。
EXPLAIN 命令用法十分简单, 在 SELECT 语句前加上 Explain 就可以了, 例如:
EXPLAIN SELECT * from user_info WHERE id < 300;
下面是我结合我自己创建的表以及执行相关sql语句总结的相关知识点。
为了接下来方便演示 EXPLAIN 的使用, 首先我们需要建立两个测试用的表, 并添加相应的数据:
DROP TABLE IF EXISTS `customers`; CREATE TABLE `customers` ( `customerNumber` int(11) NOT NULL, `customerName` varchar(50) CHARACTER SET utf8 COLLATE utf8_general_ci NOT NULL, `contactLastName` varchar(50) CHARACTER SET utf8 COLLATE utf8_general_ci NOT NULL, `contactFirstName` varchar(50) CHARACTER SET utf8 COLLATE utf8_general_ci NOT NULL, `phone` varchar(50) CHARACTER SET utf8 COLLATE utf8_general_ci NOT NULL, `addressLine1` varchar(50) CHARACTER SET utf8 COLLATE utf8_general_ci NOT NULL, `addressLine2` varchar(50) CHARACTER SET utf8 COLLATE utf8_general_ci NULL DEFAULT NULL, `city` varchar(50) CHARACTER SET utf8 COLLATE utf8_general_ci NOT NULL, `state` varchar(50) CHARACTER SET utf8 COLLATE utf8_general_ci NULL DEFAULT NULL, `postalCode` varchar(15) CHARACTER SET utf8 COLLATE utf8_general_ci NULL DEFAULT NULL, `country` varchar(50) CHARACTER SET utf8 COLLATE utf8_general_ci NOT NULL, `salesRepEmployeeNumber` int(11) NULL DEFAULT NULL, `creditLimit` decimal(10, 2) NULL DEFAULT NULL, PRIMARY KEY (`customerNumber`) USING BTREE, INDEX `salesRepEmployeeNumber`(`salesRepEmployeeNumber`) USING BTREE, INDEX `customers_idx_combine_1`(`customerName`, `phone`, `customerNumber`) USING BTREE, CONSTRAINT `customers_ibfk_1` FOREIGN KEY (`salesRepEmployeeNumber`) REFERENCES `employees` (`employeeNumber`) ON DELETE RESTRICT ON UPDATE RESTRICT ) ENGINE = InnoDB CHARACTER SET = utf8 COLLATE = utf8_general_ci ROW_FORMAT = Dynamic;
创建表成功后，插入一些测试数据。
EXPLAIN 命令的输出内容大致如下:
mysql> EXPLAIN SELECT * FROM customers WHERE customerName=‘Herkku Gifts‘ AND phone=‘+47 2267 3215‘ AND customerNumber=167; +----+-------------+-----------+------------+-------+---------------------------------+---------+---------+-------+------+----------+-------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-----------+------------+-------+---------------------------------+---------+---------+-------+------+----------+-------+ | 1 | SIMPLE | customers | NULL | const | PRIMARY,customers_idx_combine_1 | PRIMARY | 4 | const | 1 | 100.00 | NULL | +----+-------------+-----------+------------+-------+---------------------------------+---------+---------+-------+------+----------+-------+ 1 row in set, 1 warning (0.00 sec)
各列的含义如下:
接下来我们详细看一下每个字段的具体含义：
select_type 表示了查询的类型, 它的常用取值有:
SIMPLE：表示此查询不包含 UNION 查询或子查询
PRIMARY：表示此查询是最外层的查询
UNION：表示此查询是 UNION 的第二或随后的查询
DEPENDENT UNION：UNION 中的第二个或后面的查询语句, 取决于外面的查询
UNION RESULT：UNION 的结果
SUBQUERY：子查询中的第一个 SELECT
DEPENDENT SUBQUERY：子查询中的第一个 SELECT, 取决于外面的查询. 即子查询依赖于外层查询的结果
最常见的查询类别应该是 SIMPLE 了, 比如当我们的查询没有子查询, 也没有 UNION 查询时, 那么通常就是 SIMPLE 类型, 例如：
1.SIMPLE 情况：
mysql> EXPLAIN SELECT * FROM customers WHERE customerName=‘Herkku Gifts‘ AND phone=‘+47 2267 3215‘ AND customerNumber=167; +----+-------------+-----------+------------+-------+---------------------------------+---------+---------+-------+------+----------+-------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-----------+------------+-------+---------------------------------+---------+---------+-------+------+----------+-------+ | 1 | SIMPLE | customers | NULL | const | PRIMARY,customers_idx_combine_1 | PRIMARY | 4 | const | 1 | 100.00 | NULL | +----+-------------+-----------+------------+-------+---------------------------------+---------+---------+-------+------+----------+-------+ 1 row in set, 1 warning (0.00 sec)
2.UNION情况
当通过union来连接多个查询结果时，第二个之后的select其select_type为UNION
mysql> EXPLAIN SELECT customerNumber FROM customers WHERE customerNumber IN (125,144) UNION SELECT customerNumber FROM customers WHERE country IN (‘USA‘,‘France‘); +----+--------------+------------+------------+-------+---------------+---------+---------+------+------+----------+--------------------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+--------------+------------+------------+-------+---------------+---------+---------+------+------+----------+--------------------------+ | 1 | PRIMARY | customers | NULL | range | PRIMARY | PRIMARY | 4 | NULL | 2 | 100.00 | Using where; Using index | | 2 | UNION | customers | NULL | ALL | NULL | NULL | NULL | NULL | 122 | 20.00 | Using where | | NULL | UNION RESULT | <union1,2> | NULL | ALL | NULL | NULL | NULL | NULL | NULL | NULL | Using temporary | +----+--------------+------------+------------+-------+---------------+---------+---------+------+------+----------+--------------------------+ 3 rows in set, 1 warning (0.00 sec)
3.DEPENDENT UNION与DEPENDENT SUBQUERY
当union作为子查询时，其中第二个union的select_type就是DEPENDENT UNION。第一个子查询的select_type则是DEPENDENT SUBQUERY
mysql> EXPLAIN SELECT * FROM customers WHERE customerNumber IN (SELECT customerNumber FROM customers WHERE customerNumber IN (125,144) UNION SELECT customerNumber FROM customers WHERE country IN (‘USA‘,‘France‘)); +----+--------------------+------------+------------+--------+---------------+---------+---------+------+------+----------+--------------------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+--------------------+------------+------------+--------+---------------+---------+---------+------+------+----------+--------------------------+ | 1 | PRIMARY | customers | NULL | ALL | NULL | NULL | NULL | NULL | 122 | 100.00 | Using where | | 2 | DEPENDENT SUBQUERY | customers | NULL | eq_ref | PRIMARY | PRIMARY | 4 | func | 1 | 100.00 | Using where; Using index | | 3 | DEPENDENT UNION | customers | NULL | eq_ref | PRIMARY | PRIMARY | 4 | func | 1 | 20.00 | Using where | | NULL | UNION RESULT | <union2,3> | NULL | ALL | NULL | NULL | NULL | NULL | NULL | NULL | Using temporary | +----+--------------------+------------+------------+--------+---------------+---------+---------+------+------+----------+--------------------------+ 4 rows in set, 1 warning (0.00 sec)
4.SUBQUERY
子查询中的第一个select其select_type为SUBQUERY
mysql> EXPLAIN SELECT * FROM customers WHERE customerNumber=(SELECT customerNumber FROM customers WHERE customerNumber=124); +----+-------------+-----------+------------+-------+---------------+---------+---------+-------+------+----------+-------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-----------+------------+-------+---------------+---------+---------+-------+------+----------+-------------+ | 1 | PRIMARY | customers | NULL | const | PRIMARY | PRIMARY | 4 | const | 1 | 100.00 | NULL | | 2 | SUBQUERY | customers | NULL | const | PRIMARY | PRIMARY | 4 | const | 1 | 100.00 | Using index | +----+-------------+-----------+------------+-------+---------------+---------+---------+-------+------+----------+-------------+ 2 rows in set, 1 warning (0.00 sec) mysql> EXPLAIN SELECT * FROM customers WHERE customerNumber in (SELECT customerNumber FROM customers WHERE customerNumber=124); +----+-------------+-----------+------------+-------+---------------+---------+---------+-------+------+----------+-------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-----------+------------+-------+---------------+---------+---------+-------+------+----------+-------------+ | 1 | SIMPLE | customers | NULL | const | PRIMARY | PRIMARY | 4 | const | 1 | 100.00 | NULL | | 1 | SIMPLE | customers | NULL | const | PRIMARY | PRIMARY | 4 | const | 1 | 100.00 | Using index | +----+-------------+-----------+------------+-------+---------------+---------+---------+-------+------+----------+-------------+ 2 rows in set, 1 warning (0.00 sec)
思考下为什么一个用了in一个用了=反而select_type就不一样了？？？？
5.DERIVED
mysql> EXPLAIN SELECT * FROM (SELECT COUNT(*) FROM customers WHERE customerNumber=124) a; +----+-------------+------------+------------+--------+---------------+---------+---------+-------+------+----------+-------------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+------------+------------+--------+---------------+---------+---------+-------+------+----------+-------------+ | 1 | PRIMARY | <derived2> | NULL | system | NULL | NULL | NULL | NULL | 1 | 100.00 | NULL | | 2 | DERIVED | customers | NULL | const | PRIMARY | PRIMARY | 4 | const | 1 | 100.00 | Using index | +----+-------------+------------+------------+--------+---------------+---------+---------+-------+------+----------+-------------+ 2 rows in set, 1 warning (0.00 sec)
表示查询涉及的表或衍生表
匹配的分区
type 字段比较重要, 显示连接使用了何种类型。从最好到最差的连接类型依次分别为const、eq_reg、ref、range、index和ALL 它提供了判断查询是否高效的重要依据依据。
通过 type 字段, 我们判断此次查询是 全表扫描 还是 索引扫描 等。
type显示的是访问类型，是较为重要的一个指标，结果值从好到坏依次是：system > const > eq_ref > ref > fulltext > ref_or_null > index_merge > unique_subquery > index_subquery > range > index > ALL
一般来说，得保证查询至少达到range级别，最好能达到ref。
type 常用的取值有:
system: 表中只有一条数据. 这个类型是特殊的 const 类型
const: 针对主键或唯一索引的等值查询扫描, 最多只返回一行数据. const 查询速度非常快, 因为它仅仅读取一次即可。
例如下面的这个查询, 它使用了主键索引, 因此 type 就是 const 类型的
mysql> EXPLAIN SELECT * FROM customers WHERE customerName=‘Herkku Gifts‘ AND phone=‘+47 2267 3215‘ AND customerNumber=167; +----+-------------+-----------+------------+-------+---------------------------------+---------+---------+-------+------+----------+-------+ | id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-----------+------------+-------+---------------------------------+---------+---------+-------+------+----------+-------+ | 1 | SIMPLE | customers | NULL | const | PRIMARY,customers_idx_combine_1 | PRIMARY | 4 | const | 1 | 100.00 | NULL | +----+-------------+-----------+------------+-------+---------------------------------+---------+---------+-------+------+----------+-------+ 1 row in set, 1 warning (0.00 sec)
标签：使用介绍 成功 命令 xpl ddr reference cte state 接下来
原文地址：https://www.cnblogs.com/MrFiona/p/10925919.html
