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。
准备
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;
DROP TABLE IF EXISTS `employees`;
CREATE TABLE `employees` (
`employeeNumber` int(11) NOT NULL,
`lastName` varchar(50) CHARACTER SET utf8 COLLATE utf8_general_ci NOT NULL,
`firstName` varchar(50) CHARACTER SET utf8 COLLATE utf8_general_ci NOT NULL,
`extension` varchar(10) CHARACTER SET utf8 COLLATE utf8_general_ci NOT NULL,
`email` varchar(100) CHARACTER SET utf8 COLLATE utf8_general_ci NOT NULL,
`officeCode` varchar(10) CHARACTER SET utf8 COLLATE utf8_general_ci NOT NULL,
`reportsTo` int(11) NULL DEFAULT NULL,
`jobTitle` varchar(50) CHARACTER SET utf8 COLLATE utf8_general_ci NOT NULL,
PRIMARY KEY (`employeeNumber`) USING BTREE,
INDEX `reportsTo`(`reportsTo`) USING BTREE,
INDEX `officeCode`(`officeCode`) USING BTREE
) 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 输出格式
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:
- id: SELECT 查询的标识符. 每个 SELECT 都会自动分配一个唯一的标识符
- select_type: SELECT 查询的类型
- table: 查询的是哪个表
- partitions: 匹配的分区
- type: join 类型
- possible_keys: 此次查询中可能选用的索引
- key: 此次查询中确切使用到的索引
- ken_len: 表示查询优化器使用了索引的字节数
- ref: 哪个字段或常数与 key 一起被使用
- rows: 显示此查询一共扫描了多少行. 这个是一个估计值.
- filtered: 表示此查询条件所过滤的数据的百分比
- extra: 额外的信息
接下来我们详细看一下每个字段的具体含义：
select_type
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
- DERIVED：当子查询是from子句时，其select_type为DERIVED
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)
table
表示查询涉及的表或衍生表
partitions
匹配的分区
type
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。
表示通过索引一次就找到了，const用于比较primary key 或者 unique索引。因为只需匹配一行数据，所有很快。如果将主键置于where列表中，mysql就能将该查询转换为一个const 。
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 customerNumber=128;
+----+-------------+-----------+------------+-------+---------------+---------+---------+-------+------+----------+-------+
| 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 row in set, 1 warning (0.00 sec)
eq_ref：唯一性索引扫描，对于每个索引键，表中只有一条记录与之匹配。常见于主键 或 唯一索引扫描。
mysql> SELECT * FROM items;
+----+---------+
| id | item_no |
+----+---------+
| 1 | A-1 |
| 2 | A-2 |
| 3 | A-3 |
| 4 | A-4 |
| 5 | A-5 |
| 6 | A-10 |
| 7 | A-11 |
| 8 | A-20 |
| 9 | A-30 |
+----+---------+
9 rows in set (0.00 sec)
mysql> SELECT * FROM customers LIMIT 10;
+----------------+------------------------------+-----------------+------------------+-------------------+------------------------------+--------------+---------------+----------+------------+-----------+------------------------+-------------+
| customerNumber | customerName | contactLastName | contactFirstName | phone | addressLine1 | addressLine2 | city | state | postalCode | country | salesRepEmployeeNumber | creditLimit |
+----------------+------------------------------+-----------------+------------------+-------------------+------------------------------+--------------+---------------+----------+------------+-----------+------------------------+-------------+
| 103 | Atelier graphique | Schmitt | Carine | 40.32.2555 | 54, rue Royale | NULL | Nantes | NULL | 44000 | France | 1370 | 21000.00 |
| 112 | Signal Gift Stores | King | Jean | 7025551838 | 8489 Strong St. | NULL | Las Vegas | NV | 83030 | USA | 1166 | 71800.00 |
| 114 | Australian Collectors, Co. | Ferguson | Peter | 03 9520 4555 | 636 St Kilda Road | Level 3 | Melbourne | Victoria | 3004 | Australia | 1611 | 117300.00 |
| 119 | La Rochelle Gifts | Labrune | Janine | 40.67.8555 | 67, rue des Cinquante Otages | NULL | Nantes | NULL | 44000 | France | 1370 | 118200.00 |
| 121 | Baane Mini Imports | Bergulfsen | Jonas | 07-98 9555 | Erling Skakkes gate 78 | NULL | Stavern | NULL | 4110 | Norway | 1504 | 81700.00 |
| 124 | Mini Gifts Distributors Ltd. | Nelson | Susan | 4155551450 | 5677 Strong St. | NULL | San Rafael | CA | 97562 | USA | 1165 | 210500.00 |
| 125 | Havel & Zbyszek Co | Piestrzeniewicz | Zbyszek | (26) 642-7555 | ul. Filtrowa 68 | NULL | Warszawa | NULL | 01-012 | Poland | NULL | 0.00 |
| 128 | Blauer See Auto, Co. | Keitel | Roland | +49 69 66 90 2555 | Lyonerstr. 34 | NULL | Frankfurt | NULL | 60528 | Germany | 1504 | 59700.00 |
| 129 | Mini Wheels Co. | Murphy | Julie | 6505555787 | 5557 North Pendale Street | NULL | San Francisco | CA | 94217 | USA | 1165 | 64600.00 |
| 131 | Land of Toys Inc. | Lee | Kwai | 2125557818 | 897 Long Airport Avenue | NULL | NYC | NY | 10022 | USA | 1323 | 114900.00 |
+----------------+------------------------------+-----------------+------------------+-------------------+------------------------------+--------------+---------------+----------+------------+-----------+------------------------+-------------+
10 rows in set (0.00 sec)
mysql> EXPLAIN SELECT * FROM items,customers WHERE customers.customerNumber=items.id;
+----+-------------+-----------+------------+--------+---------------+---------+---------+-------------------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-----------+------------+--------+---------------+---------+---------+-------------------+------+----------+-------+
| 1 | SIMPLE | items | NULL | ALL | PRIMARY | NULL | NULL | NULL | 9 | 100.00 | NULL |
| 1 | SIMPLE | customers | NULL | eq_ref | PRIMARY | PRIMARY | 4 | yiibaidb.items.id | 1 | 100.00 | NULL |
+----+-------------+-----------+------------+--------+---------------+---------+---------+-------------------+------+----------+-------+
2 rows in set, 1 warning (0.00 sec)
注意：ALL全表扫描的表是记录最少的表如items表。
ref：非唯一性索引扫描，返回匹配某个单独值的所有行。本质是也是一种索引访问，它返回所有匹配某个单独值的行，然而他可能会找到多个符合条件的行，所以它应该属于查找和扫描的混合体 。
mysql> SELECT * FROM customers WHERE salesRepEmployeeNumber=1165;
+----------------+------------------------------+-----------------+------------------+------------+---------------------------+--------------+---------------+-------+------------+---------+------------------------+-------------+
| customerNumber | customerName | contactLastName | contactFirstName | phone | addressLine1 | addressLine2 | city | state | postalCode | country | salesRepEmployeeNumber | creditLimit |
+----------------+------------------------------+-----------------+------------------+------------+---------------------------+--------------+---------------+-------+------------+---------+------------------------+-------------+
| 124 | Mini Gifts Distributors Ltd. | Nelson | Susan | 4155551450 | 5677 Strong St. | NULL | San Rafael | CA | 97562 | USA | 1165 | 210500.00 |
| 129 | Mini Wheels Co. | Murphy | Julie | 6505555787 | 5557 North Pendale Street | NULL | San Francisco | CA | 94217 | USA | 1165 | 64600.00 |
| 161 | Technics Stores Inc. | Hashimoto | Juri | 6505556809 | 9408 Furth Circle | NULL | Burlingame | CA | 94217 | USA | 1165 | 84600.00 |
| 321 | Corporate Gift Ideas Co. | Brown | Julie | 6505551386 | 7734 Strong St. | NULL | San Francisco | CA | 94217 | USA | 1165 | 105000.00 |
| 450 | The Sharp Gifts Warehouse | Frick | Sue | 4085553659 | 3086 Ingle Ln. | NULL | San Jose | CA | 94217 | USA | 1165 | 77600.00 |
| 487 | Signal Collectibles Ltd. | Taylor | Sue | 4155554312 | 2793 Furth Circle | NULL | Brisbane | CA | 94217 | USA | 1165 | 60300.00 |
+----------------+------------------------------+-----------------+------------------+------------+---------------------------+--------------+---------------+-------+------------+---------+------------------------+-------------+
6 rows in set (0.00 sec)mysql> EXPLAIN SELECT * FROM customers WHERE salesRepEmployeeNumber=1165;
+----+-------------+-----------+------------+------+------------------------+------------------------+---------+-------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-----------+------------+------+------------------------+------------------------+---------+-------+------+----------+-------+
| 1 | SIMPLE | customers | NULL | ref | salesRepEmployeeNumber | salesRepEmployeeNumber | 5 | const | 6 | 100.00 | NULL |
+----+-------------+-----------+------------+------+------------------------+------------------------+---------+-------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)
mysql> show index from customers;
+-----------+------------+-------------------------+--------------+------------------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+-----------+------------+-------------------------+--------------+------------------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| customers | 0 | PRIMARY | 1 | customerNumber | A | 122 | NULL | NULL | | BTREE | | |
| customers | 1 | salesRepEmployeeNumber | 1 | salesRepEmployeeNumber | A | 16 | NULL | NULL | YES | BTREE | | |
| customers | 1 | customers_idx_combine_1 | 1 | customerName | A | 122 | NULL | NULL | | BTREE | | |
| customers | 1 | customers_idx_combine_1 | 2 | phone | A | 122 | NULL | NULL | | BTREE | | |
| customers | 1 | customers_idx_combine_1 | 3 | customerNumber | A | 122 | NULL | NULL | | BTREE | | |
+-----------+------------+-------------------------+--------------+------------------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
5 rows in set (0.00 sec)
range：只检索给定范围的行，使用一个索引来选择行。key列显示使用了那个索引。一般就是在where语句中出现了bettween、<、>、in等的查询。这种索引列上的范围扫描比全索引扫描要好。只需要开始于某个点，结束于另一个点，不用扫描全部索引 。
mysql> EXPLAIN SELECT * FROM customers WHERE customerNumber BETWEEN 1 AND 120;
+----+-------------+-----------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-----------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
| 1 | SIMPLE | customers | NULL | range | PRIMARY | PRIMARY | 4 | NULL | 4 | 100.00 | Using where |
+----+-------------+-----------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)
index：Full Index Scan，index与ALL区别为index类型只遍历索引树。这通常为ALL快，因为索引文件通常比数据文件小。（Index与ALL虽然都是读全表，但index是从索引中读取，而ALL是从硬盘读取） 。
mysql> EXPLAIN SELECT id FROM items;
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
| 1 | SIMPLE | items | NULL | index | NULL | PRIMARY | 4 | NULL | 9 | 100.00 | Using index |
+----+-------------+-------+------------+-------+---------------+---------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)mysql> show index from items;
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| items | 0 | PRIMARY | 1 | id | A | 9 | NULL | NULL | | BTREE | | |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
1 row in set (0.00 sec)
ALL：Full Table Scan，遍历全表以找到匹配的行 。
mysql> EXPLAIN SELECT item_no FROM items;
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------+
| 1 | SIMPLE | items | NULL | ALL | NULL | NULL | NULL | NULL | 9 | 100.00 | NULL |
+----+-------------+-------+------------+------+---------------+------+---------+------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)mysql> show index from items;
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| items | 0 | PRIMARY | 1 | id | A | 9 | NULL | NULL | | BTREE | | |
+-------+------------+----------+--------------+-------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
1 row in set (0.00 sec)
possible_keys
查询涉及到的字段上存在索引，则该索引将被列出，但不一定被查询实际使用。
key
实际使用的索引，如果为NULL，则没有使用索引。
查询中如果使用了覆盖索引，则该索引仅出现在key列表中。
mysql> show index from customers;
+-----------+------------+-------------------------+--------------+------------------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| Table | Non_unique | Key_name | Seq_in_index | Column_name | Collation | Cardinality | Sub_part | Packed | Null | Index_type | Comment | Index_comment |
+-----------+------------+-------------------------+--------------+------------------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
| customers | 0 | PRIMARY | 1 | customerNumber | A | 122 | NULL | NULL | | BTREE | | |
| customers | 1 | salesRepEmployeeNumber | 1 | salesRepEmployeeNumber | A | 16 | NULL | NULL | YES | BTREE | | |
| customers | 1 | customers_idx_combine_1 | 1 | customerName | A | 122 | NULL | NULL | | BTREE | | |
| customers | 1 | customers_idx_combine_1 | 2 | phone | A | 122 | NULL | NULL | | BTREE | | |
| customers | 1 | customers_idx_combine_1 | 3 | customerNumber | A | 122 | NULL | NULL | | BTREE | | |
+-----------+------------+-------------------------+--------------+------------------------+-----------+-------------+----------+--------+------+------------+---------+---------------+
5 rows in set (0.00 sec)mysql> EXPLAIN SELECT customerName,phone,customerNumber from customers;
+----+-------------+-----------+------------+-------+---------------+-------------------------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-----------+------------+-------+---------------+-------------------------+---------+------+------+----------+-------------+
| 1 | SIMPLE | customers | NULL | index | NULL | customers_idx_combine_1 | 308 | NULL | 122 | 100.00 | Using index |
+----+-------------+-----------+------------+-------+---------------+-------------------------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)
上述customerName,phone,customerNumber与customers_idx_combine_1索引顺序一致，为覆盖索引
mysql> EXPLAIN SELECT phone,customerName,customerNumber from customers;
+----+-------------+-----------+------------+-------+---------------+-------------------------+---------+------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-----------+------------+-------+---------------+-------------------------+---------+------+------+----------+-------------+
| 1 | SIMPLE | customers | NULL | index | NULL | customers_idx_combine_1 | 308 | NULL | 122 | 100.00 | Using index |
+----+-------------+-----------+------------+-------+---------------+-------------------------+---------+------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)
mysql> EXPLAIN SELECT customerName,phone,customerNumber,state from customers;
+----+-------------+-----------+------------+------+---------------+------+---------+------+------+----------+-------+
| id | select_type | table | partitions | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-----------+------------+------+---------------+------+---------+------+------+----------+-------+
| 1 | SIMPLE | customers | NULL | ALL | NULL | NULL | NULL | NULL | 122 | 100.00 | NULL |
+----+-------------+-----------+------------+------+---------------+------+---------+------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)
state字段为非索引列。
key_len
表示索引中使用的字节数，查询中使用的索引的长度（最大可能长度），并非实际使用长度，理论上长度越短越好。key_len是根据表定义计算而得的，不是通过表内检索出的。
ref
显示索引的那一列被使用了，如果可能，是一个常量const。
mysql> EXPLAIN SELECT customerNumber FROM customers WHERE customerNumber=114;
+----+-------------+-----------+------------+-------+---------------+---------+---------+-------+------+----------+-------------+
| 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 | Using index |
+----+-------------+-----------+------------+-------+---------------+---------+---------+-------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)
rows
根据表统计信息及索引选用情况，大致估算出找到所需的记录所需要读取的行数。
Extra
不适合在其他字段中显示，但是十分重要的额外信息。
Using filesort :mysql对数据使用一个外部的索引排序，而不是按照表内的索引进行排序读取。也就是说mysql无法利用索引完成的排序操作成为“文件排序” 。
