MySQL执行计划explain的key_len解析
内容导读
互联网集市收集整理的这篇技术教程文章主要介绍了MySQL执行计划explain的key_len解析，小编现在分享给大家，供广大互联网技能从业者学习和参考。文章包含30955字，纯文字阅读大概需要45分钟。
内容图文
.0.1 [mytest]> desc table_key;
+---------+-------------+------+-----+---------+-------+
| Field | Type | Null | Key | Default | Extra |
+---------+-------------+------+-----+---------+-------+
| id | int(11) | NO | | NULL | |
| sid | bigint(20) | NO | | 0 | |
| name | char(10) | YES | | NULL | |
| age | tinyint(4) | YES | | NULL | |
| sex | tinyint(4) | NO | | NULL | |
| address | varchar(10) | YES | MUL | NULL | |
+---------+-------------+------+-----+---------+-------+
6 rows in set (0.01 sec)
(dg1)root@127.0.0.1 [mytest]>
来看看tinyint类型的索引长度，在NOT NULL 和 NULL 的时候
(dg1)root@127.0.0.1 [mytest]> explain extended select * from table_key where age=38;
+----+-------------+-----------+------+---------------+-----------+---------+-------+------+----------+-------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-----------+------+---------------+-----------+---------+-------+------+----------+-------+
| 1 | SIMPLE | table_key | ref | age_index | age_index | 1 | const | 1 | 100.00 | NULL |
+----+-------------+-----------+------+---------------+-----------+---------+-------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)
(dg1)root@127.0.0.1 [mytest]> alter table table_key modify age tinyint(4);
(dg1)root@127.0.0.1 [mytest]> explain extended select * from table_key where age=38;
+----+-------------+-----------+------+---------------+-----------+---------+-------+------+----------+-------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | filtered | Extra |
+----+-------------+-----------+------+---------------+-----------+---------+-------+------+----------+-------+
| 1 | SIMPLE | table_key | ref | age_index | age_index | 2 | const | 1 | 100.00 | NULL |
+----+-------------+-----------+------+---------------+-----------+---------+-------+------+----------+-------+
1 row in set, 1 warning (0.00 sec)
(dg1)root@127.0.0.1 [mytest]>
看看bigint类型的索引长度，同样是 NOT NULL 和 NULL值的时候，分别是1和2
(dg1)root@127.0.0.1 [mytest]> alter table table_key add key sid_index (sid);
Query OK, 0 rows affected (0.04 sec)
Records: 0 Duplicates: 0 Warnings: 0
(dg1)root@127.0.0.1 [mytest]> desc select * from table_key where sid=6;
+----+-------------+-----------+------+---------------+-----------+---------+-------+------+-------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-----------+------+---------------+-----------+---------+-------+------+-------+
| 1 | SIMPLE | table_key | ref | sid_index | sid_index | 8 | const | 1 | NULL |
+----+-------------+-----------+------+---------------+-----------+---------+-------+------+-------+
1 row in set (0.00 sec)
(dg1)root@127.0.0.1 [mytest]>
(dg1)root@127.0.0.1 [mytest]> alter table table_key modify sid bigint(20);
Query OK, 0 rows affected (0.08 sec)
Records: 0 Duplicates: 0 Warnings: 0
(dg1)root@127.0.0.1 [mytest]> desc select * from table_key where sid=6;
+----+-------------+-----------+------+---------------+-----------+---------+-------+------+-------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-----------+------+---------------+-----------+---------+-------+------+-------+
| 1 | SIMPLE | table_key | ref | sid_index | sid_index | 9 | const | 1 | NULL |
+----+-------------+-----------+------+---------------+-----------+---------+-------+------+-------+
1 row in set (0.00 sec)
看看smallint类型索引长度，同样是 NOT NULL 和 NULL值的时候，分别是2和3(dg1)root@127.0.0.1 [mytest]> alter table table_key modify sid smallint not null default 0; Query OK, 9 rows affected (0.04 sec) Records: 9 Duplicates: 0 Warnings: 0 (dg1)root@127.0.0.1 [mytest]> desc select * from table_key where sid=6; +----+-------------+-----------+------+---------------+-----------+---------+-------+------+-------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-----------+------+---------------+-----------+---------+-------+------+-------+ | 1 | SIMPLE | table_key | ref | sid_index | sid_index | 2 | const | 1 | NULL | +----+-------------+-----------+------+---------------+-----------+---------+-------+------+-------+ 1 row in set (0.00 sec) (dg1)root@127.0.0.1 [mytest]> alter table table_key modify sid smallint ; Query OK, 0 rows affected (0.07 sec) Records: 0 Duplicates: 0 Warnings: 0 (dg1)root@127.0.0.1 [mytest]> desc select * from table_key where sid=6; +----+-------------+-----------+------+---------------+-----------+---------+-------+------+-------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-----------+------+---------------+-----------+---------+-------+------+-------+ | 1 | SIMPLE | table_key | ref | sid_index | sid_index | 3 | const | 1 | NULL | +----+-------------+-----------+------+---------------+-----------+---------+-------+------+-------+ 1 row in set (0.00 sec) 看看mediumint类型索引长度，同样是 NOT NULL 和 NULL值的时候，分别是3和4 (dg1)root@127.0.0.1 [mytest]> alter table table_key modify sid mediumint NOT NULL; Query OK, 0 rows affected (0.06 sec) Records: 0 Duplicates: 0 Warnings: 0 (dg1)root@127.0.0.1 [mytest]> desc select * from table_key where sid=6; +----+-------------+-----------+------+---------------+-----------+---------+-------+------+-------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+-----------+------+---------------+-----------+---------+-------+------+-------+ | 1 | SIMPLE | table_key | ref | sid_index | sid_index | 3 | const | 1 | NULL | +----+-------------+-----------+------+---------------+-----------+---------+-------+------+-------+ 1 row in set (0.00 sec) (dg1)root@127.0.0.1 [mytest]>
（dg1)root@127.0.0.1 [mytest]> alter table table_key modify sid mediumint ;
Query OK, 0 rows affected (0.06 sec)
Records: 0 Duplicates: 0 Warnings: 0
(dg1)root@127.0.0.1 [mytest]> desc select * from table_key where sid=6;
+----+-------------+-----------+------+---------------+-----------+---------+-------+------+-------+
| id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra |
+----+-------------+-----------+------+---------------+-----------+---------+-------+------+-------+
| 1 | SIMPLE | table_key | ref | sid_index | sid_index | 4 | const | 1 | NULL |
+----+-------------+-----------+------+---------------+-----------+---------+-------+------+-------+
1 row in set (0.00 sec)
(dg1)root@127.0.0.1 [mytest]>
2.浮点数类型
表结构
CREATE TABLE `table_key1` ( `id` int NOT NULL AUTO_INCREMENT , `c1` float NOT NULL , `c2` double NOT NULL , `c3` decimal NOT NULL , `c4` date NOT NULL , `c5` timestamp NOT NULL ON UPDATE CURRENT_TIMESTAMP , `c6` datetime NOT NULL , PRIMARY KEY (`id`) )
看看float类型的索引长度，NOT NULL和NULL的时候，分别是4和5 (dg1)root@127.0.0.1 [mytest]> desc select * from table_key1 where c1 = ‘3.22‘; +----+-------------+------------+------+---------------+----------+---------+-------+------+-----------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+------------+------+---------------+----------+---------+-------+------+-----------------------+ | 1 | SIMPLE | table_key1 | ref | c1_index | c1_index | 4 | const | 8 | Using index condition | +----+-------------+------------+------+---------------+----------+---------+-------+------+-----------------------+ 1 row in set (0.00 sec) (dg1)root@127.0.0.1 [mytest]> alter table table_key1 modify c1 float; Query OK, 0 rows affected (0.05 sec) Records: 0 Duplicates: 0 Warnings: 0 (dg1)root@127.0.0.1 [mytest]> desc select * from table_key1 where c1 = ‘3.22‘; +----+-------------+------------+------+---------------+----------+---------+-------+------+-----------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+------------+------+---------------+----------+---------+-------+------+-----------------------+ | 1 | SIMPLE | table_key1 | ref | c1_index | c1_index | 5 | const | 8 | Using index condition | +----+-------------+------------+------+---------------+----------+---------+-------+------+-----------------------+ 1 row in set (0.00 sec) 看看double类型的索引长度，NOT NULL和NULL的时候，分别是8和9 (dg1)root@127.0.0.1 [mytest]> alter table table_key1 add key c2_index (c2); Query OK, 0 rows affected (0.04 sec) Records: 0 Duplicates: 0 Warnings: 0 (dg1)root@127.0.0.1 [mytest]> desc select * from table_key1 where c2 = ‘3.22‘; +----+-------------+------------+------+---------------+----------+---------+-------+------+-------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+------------+------+---------------+----------+---------+-------+------+-------+ | 1 | SIMPLE | table_key1 | ref | c2_index | c2_index | 8 | const | 1 | NULL | +----+-------------+------------+------+---------------+----------+---------+-------+------+-------+ 1 row in set (0.00 sec) (dg1)root@127.0.0.1 [mytest]> alter table table_key1 modify c2 double; Query OK, 0 rows affected (0.03 sec) Records: 0 Duplicates: 0 Warnings: 0 (dg1)root@127.0.0.1 [mytest]> desc select * from table_key1 where c2 = ‘3.22‘; +----+-------------+------------+------+---------------+----------+---------+-------+------+-------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+------------+------+---------------+----------+---------+-------+------+-------+ | 1 | SIMPLE | table_key1 | ref | c2_index | c2_index | 9 | const | 1 | NULL | +----+-------------+------------+------+---------------+----------+---------+-------+------+-------+ 1 row in set (0.00 sec) (dg1)root@127.0.0.1 [mytest]>
3、看看时间类型
看看date类型的索引长度，在NOT NULL和NULL的时候，分别是3和4
(dg1)root@127.0.0.1 [mytest]> desc select * from table_key1 where c4 = ‘2015-05-06‘; +----+-------------+------------+------+---------------+----------+---------+-------+------+-----------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+------------+------+---------------+----------+---------+-------+------+-----------------------+ | 1 | SIMPLE | table_key1 | ref | c4_index | c4_index | 3 | const | 4 | Using index condition | +----+-------------+------------+------+---------------+----------+---------+-------+------+-----------------------+ 1 row in set, 3 warnings (0.00 sec) (dg1)root@127.0.0.1 [mytest]> (dg1)root@127.0.0.1 [mytest]> alter table table_key1 modify c4 date; Query OK, 0 rows affected (0.09 sec) Records: 0 Duplicates: 0 Warnings: 0 (dg1)root@127.0.0.1 [mytest]> desc select * from table_key1 where c4 = ‘2015-05-06‘; +----+-------------+------------+------+---------------+----------+---------+-------+------+-----------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+------------+------+---------------+----------+---------+-------+------+-----------------------+ | 1 | SIMPLE | table_key1 | ref | c4_index | c4_index | 4 | const | 4 | Using index condition | +----+-------------+------------+------+---------------+----------+---------+-------+------+-----------------------+ 1 row in set, 3 warnings (0.00 sec) (dg1)root@127.0.0.1 [mytest]> 在timestamp类型的时候索引长度，在NOT NULL 和 NULL的时候，分别是4和5 (dg1)root@127.0.0.1 [mytest]> desc select * from table_key1 where c5 = ‘2015-05-06 11:23:21‘ ; +----+-------------+------------+------+---------------+----------+---------+-------+------+-----------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+------------+------+---------------+----------+---------+-------+------+-----------------------+ | 1 | SIMPLE | table_key1 | ref | c5_index | c5_index | 4 | const | 5 | Using index condition | +----+-------------+------------+------+---------------+----------+---------+-------+------+-----------------------+ 1 row in set, 3 warnings (0.00 sec) (dg1)root@127.0.0.1 [mytest]> dg1)root@127.0.0.1 [mytest]> alter table table_key1 modify c5 timestamp ON UPDATE CURRENT_TIMESTAMP ; Query OK, 0 rows affected (0.06 sec) Records: 0 Duplicates: 0 Warnings: 0 (dg1)root@127.0.0.1 [mytest]> (dg1)root@127.0.0.1 [mytest]> desc select * from table_key1 where c5 = ‘2015-05-06 110:23:21‘; +----+-------------+------------+------+---------------+----------+---------+-------+------+-----------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+------------+------+---------------+----------+---------+-------+------+-----------------------+ | 1 | SIMPLE | table_key1 | ref | c5_index | c5_index | 5 | const | 5 | Using index condition | +----+-------------+------------+------+---------------+----------+---------+-------+------+-----------------------+ 1 row in set, 3 warnings (0.00 sec) (dg1)root@127.0.0.1 [mytest]> (dg1)root@127.0.0.1 [mytest]> desc select * from table_key1 where c6 = ‘2015-05-06 11:23:38‘; +----+-------------+------------+------+---------------+----------+---------+-------+------+-----------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+------------+------+---------------+----------+---------+-------+------+-----------------------+ | 1 | SIMPLE | table_key1 | ref | c6_index | c6_index | 8 | const | 4 | Using index condition | +----+-------------+------------+------+---------------+----------+---------+-------+------+-----------------------+ 1 row in set, 3 warnings (0.00 sec) (dg1)root@127.0.0.1 [mytest]> (dg1)root@127.0.0.1 [mytest]> alter table table_key1 modify c6 datetime ; Query OK, 0 rows affected (0.08 sec) Records: 0 Duplicates: 0 Warnings: 0 (dg1)root@127.0.0.1 [mytest]> desc select * from table_key1 where c6 = ‘2015-05-06 11:23:38‘; +----+-------------+------------+------+---------------+----------+---------+-------+------+-----------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | Extra | +----+-------------+------------+------+---------------+----------+---------+-------+------+-----------------------+ | 1 | SIMPLE | table_key1 | ref | c6_index | c6_index | 9 | const | 4 | Using index condition | +----+-------------+------------+------+---------------+----------+---------+-------+------+-----------------------+ 1 row in set, 3 warnings (0.00 sec)
4.字符类型
表结构，字符集是UTF8 (dg1)root@127.0.0.1 [mytest]> desc table_key; +---------+-------------+------+-----+---------+-------+ | Field | Type | Null | Key | Default | Extra | +---------+-------------+------+-----+---------+-------+ | id | int(11) | NO | | NULL | | | sid | bigint(20) | NO | | 0 | | | name | char(10) | YES | | NULL | | | age | tinyint(4) | YES | | NULL | | | sex | tinyint(4) | NO | | NULL | | | address | varchar(10) | YES | MUL | NULL | | +---------+-------------+------+-----+---------+-------+ 6 rows in set (0.01 sec) 看看定长字符类型char的索引长度，在NOT NULL 和NULL中分别为10*3和10*3+1 (dg1)root@127.0.0.1 [mytest]> alter table table_key add index name_index (name); Query OK, 0 rows affected (0.01 sec) Records: 0 Duplicates: 0 Warnings: 0 (dg1)root@127.0.0.1 [mytest]> explain extended select * from table_key where name=‘zhangsan‘; +----+-------------+-----------+------+---------------+------------+---------+-------+------+----------+-----------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-----------+------+---------------+------------+---------+-------+------+----------+-----------------------+ | 1 | SIMPLE | table_key | ref | name_index | name_index | 30 | const | 1 | 100.00 | Using index condition | +----+-------------+-----------+------+---------------+------------+---------+-------+------+----------+-----------------------+ 1 row in set, 1 warning (0.00 sec) (dg1)root@127.0.0.1 [mytest]> (dg1)root@127.0.0.1 [mytest]> alter table table_key modify name char(10); Query OK, 0 rows affected (0.05 sec) Records: 0 Duplicates: 0 Warnings: 0 (dg1)root@127.0.0.1 [mytest]> explain extended select * from table_key where name=‘zhangsan‘; +----+-------------+-----------+------+---------------+------------+---------+-------+------+----------+-----------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-----------+------+---------------+------------+---------+-------+------+----------+-----------------------+ | 1 | SIMPLE | table_key | ref | name_index | name_index | 31 | const | 1 | 100.00 | Using index condition | +----+-------------+-----------+------+---------------+------------+---------+-------+------+----------+-----------------------+ 1 row in set, 1 warning (0.00 sec) 看看变长长字符类型varchar的索引长度，在NOT NULL 和NULL中分别为10*3+2和10*3+2+1 (dg1)root@127.0.0.1 [mytest]> explain extended select * from table_key where address=‘shanghai‘; +----+-------------+-----------+------+---------------+---------------+---------+-------+------+----------+-----------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-----------+------+---------------+---------------+---------+-------+------+----------+-----------------------+ | 1 | SIMPLE | table_key | ref | address_index | address_index | 32 | const | 1 | 100.00 | Using index condition | +----+-------------+-----------+------+---------------+---------------+---------+-------+------+----------+-----------------------+ 1 row in set, 1 warning (0.01 sec) (dg1)root@127.0.0.1 [mytest]> (dg1)root@127.0.0.1 [mytest]> alter table table_key modify address varchar(10); Query OK, 0 rows affected (0.10 sec) Records: 0 Duplicates: 0 Warnings: 0 (dg1)root@127.0.0.1 [mytest]> explain extended select * from table_key where address=‘shanghai‘; +----+-------------+-----------+------+---------------+---------------+---------+-------+------+----------+-----------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-----------+------+---------------+---------------+---------+-------+------+----------+-----------------------+ | 1 | SIMPLE | table_key | ref | address_index | address_index | 33 | const | 1 | 100.00 | Using index condition | +----+-------------+-----------+------+---------------+---------------+---------+-------+------+----------+-----------------------+ 1 row in set, 1 warning (0.00 sec) (dg1)root@127.0.0.1 [mytest]>
修改字符集为GBK，再来看看 (dg1)root@127.0.0.1 [mytest]> ALTER TABLE `table_key` DEFAULT CHARACTER SET=gbk COLLATE=gbk_chinese_ci; (dg1)root@127.0.0.1 [mytest]> ALTER database `mytest` DEFAULT CHARACTER SET=gbk COLLATE=gbk_chinese_ci; (dg1)root@127.0.0.1 [mytest]> show global variables like ‘%char%‘; +--------------------------+--------------------------------+ | Variable_name | Value | +--------------------------+--------------------------------+ | character_set_client | gbk | | character_set_connection | gbk | | character_set_database | gbk | | character_set_filesystem | binary | | character_set_results | gbk | | character_set_server | gbk | | character_set_system | utf8 | | character_sets_dir | /opt/app/mysql/share/charsets/ | +--------------------------+--------------------------------+ 8 rows in set (0.00 sec) 奇怪了，为什么还是10*3+2+1呢，是因为字段的字符集还没修改过来 (dg1)root@127.0.0.1 [mytest]> explain extended select * from table_key where address=‘shanghai‘; +----+-------------+-----------+------+---------------+---------------+---------+-------+------+----------+-----------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-----------+------+---------------+---------------+---------+-------+------+----------+-----------------------+ | 1 | SIMPLE | table_key | ref | address_index | address_index | 33 | const | 1 | 100.00 | Using index condition | +----+-------------+-----------+------+---------------+---------------+---------+-------+------+----------+-----------------------+ 1 row in set, 1 warning (0.01 sec) (dg1)root@127.0.0.1 [mytest]> ALTER TABLE `table_key` MODIFY COLUMN `name` char(10) CHARACTER SET gbk COLLATE gbk_chinese_ci NULL DEFAULT NULL , DEFAULT CHARACTER SET=gbk COLLATE=gbk_chinese_ci; Query OK, 9 rows affected (0.04 sec) Records: 9 Duplicates: 0 Warnings: 0 (dg1)root@127.0.0.1 [mytest]> ALTER TABLE `table_key` MODIFY COLUMN `address` char(10) CHARACTER SET gbk COLLATE gbk_chinese_ci NULL DEFAULT NULL , DEFAULT CHARACTER SET=gbk COLLATE=gbk_chinese_ci; Query OK, 9 rows affected (0.03 sec) Records: 9 Duplicates: 0 Warnings: 0 (dg1)root@127.0.0.1 [mytest]> explain extended select * from table_key where address=‘shanghai‘; +----+-------------+-----------+------+---------------+---------------+---------+-------+------+----------+-----------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-----------+------+---------------+---------------+---------+-------+------+----------+-----------------------+ | 1 | SIMPLE | table_key | ref | address_index | address_index | 21 | const | 1 | 100.00 | Using index condition | +----+-------------+-----------+------+---------------+---------------+---------+-------+------+----------+-----------------------+ 1 row in set, 1 warning (0.00 sec) (dg1)root@127.0.0.1 [mytest]> (dg1)root@127.0.0.1 [mytest]> alter table table_key modify address varchar(10); Query OK, 9 rows affected (0.04 sec) Records: 9 Duplicates: 0 Warnings: 0 (dg1)root@127.0.0.1 [mytest]> explain extended select * from table_key where address=‘shanghai‘; +----+-------------+-----------+------+---------------+---------------+---------+-------+------+----------+-----------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-----------+------+---------------+---------------+---------+-------+------+----------+-----------------------+ | 1 | SIMPLE | table_key | ref | address_index | address_index | 23 | const | 1 | 100.00 | Using index condition | +----+-------------+-----------+------+---------------+---------------+---------+-------+------+----------+-----------------------+ 1 row in set, 1 warning (0.01 sec) (dg1)root@127.0.0.1 [mytest]> (dg1)root@127.0.0.1 [mytest]> explain extended select * from table_key where name=‘zhangsan‘; +----+-------------+-----------+------+---------------+------------+---------+-------+------+----------+-----------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-----------+------+---------------+------------+---------+-------+------+----------+-----------------------+ | 1 | SIMPLE | table_key | ref | name_index | name_index | 20 | const | 1 | 100.00 | Using index condition | +----+-------------+-----------+------+---------------+------------+---------+-------+------+----------+-----------------------+ 1 row in set, 1 warning (0.00 sec) (dg1)root@127.0.0.1 [mytest]> (dg1)root@127.0.0.1 [mytest]> alter table table_key modify name char(10); Query OK, 0 rows affected (0.00 sec) Records: 0 Duplicates: 0 Warnings: 0 (dg1)root@127.0.0.1 [mytest]> explain extended select * from table_key where name=‘zhangsan‘; +----+-------------+-----------+------+---------------+------------+---------+-------+------+----------+-----------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-----------+------+---------------+------------+---------+-------+------+----------+-----------------------+ | 1 | SIMPLE | table_key | ref | name_index | name_index | 21 | const | 1 | 100.00 | Using index condition | +----+-------------+-----------+------+---------------+------------+---------+-------+------+----------+-----------------------+ 1 row in set, 1 warning (0.01 sec)
(dg1)root@127.0.0.1 [mytest]> alter table table_key drop index name_index; Query OK, 0 rows affected (0.01 sec) Records: 0 Duplicates: 0 Warnings: 0 (dg1)root@127.0.0.1 [mytest]> alter table table_key drop index address_index; Query OK, 0 rows affected (0.01 sec) Records: 0 Duplicates: 0 Warnings: 0 (dg1)root@127.0.0.1 [mytest]> alter table table_key add index name_address_index (name,address); Query OK, 0 rows affected (0.02 sec) Records: 0 Duplicates: 0 Warnings: 0 (dg1)root@127.0.0.1 [mytest]> explain extended select * from table_key where address=‘shanghai‘ and name=‘zhangsan‘; +----+-------------+-----------+------+--------------------+--------------------+---------+-------------+------+----------+-----------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-----------+------+--------------------+--------------------+---------+-------------+------+----------+-----------------------+ | 1 | SIMPLE | table_key | ref | name_address_index | name_address_index | 42 | const,const | 1 | 100.00 | Using index condition | +----+-------------+-----------+------+--------------------+--------------------+---------+-------------+------+----------+-----------------------+ 1 row in set, 1 warning (0.00 sec) (dg1)root@127.0.0.1 [mytest]> (dg1)root@127.0.0.1 [mytest]> explain extended select * from table_key where name=‘zhangsan‘ and address=‘shanghai‘; +----+-------------+-----------+------+--------------------+--------------------+---------+-------------+------+----------+-----------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-----------+------+--------------------+--------------------+---------+-------------+------+----------+-----------------------+ | 1 | SIMPLE | table_key | ref | name_address_index | name_address_index | 43 | const,const | 1 | 100.00 | Using index condition | +----+-------------+-----------+------+--------------------+--------------------+---------+-------------+------+----------+-----------------------+ 1 row in set, 1 warning (0.00 sec) (dg1)root@127.0.0.1 [mytest]> alter table table_key modify name char(10); Query OK, 0 rows affected (0.03 sec) Records: 0 Duplicates: 0 Warnings: 0 (dg1)root@127.0.0.1 [mytest]> explain extended select * from table_key where name=‘zhangsan‘; +----+-------------+-----------+------+--------------------+--------------------+---------+-------+------+----------+-----------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-----------+------+--------------------+--------------------+---------+-------+------+----------+-----------------------+ | 1 | SIMPLE | table_key | ref | name_address_index | name_address_index | 21 | const | 1 | 100.00 | Using index condition | +----+-------------+-----------+------+--------------------+--------------------+---------+-------+------+----------+-----------------------+ 1 row in set, 1 warning (0.00 sec) (dg1)root@127.0.0.1 [mytest]> alter table table_key modify address varchar(10); Query OK, 0 rows affected (0.04 sec) Records: 0 Duplicates: 0 Warnings: 0 (dg1)root@127.0.0.1 [mytest]> explain extended select * from table_key where name=‘zhangsan‘ and address=‘shanghai‘; +----+-------------+-----------+------+--------------------+--------------------+---------+-------------+------+----------+-----------------------+ | id | select_type | table | type | possible_keys | key | key_len | ref | rows | filtered | Extra | +----+-------------+-----------+------+--------------------+--------------------+---------+-------------+------+----------+-----------------------+ | 1 | SIMPLE | table_key | ref | name_address_index | name_address_index | 44 | const,const | 1 | 100.00 | Using index condition | +----+-------------+-----------+------+--------------------+--------------------+---------+-------------+------+----------+-----------------------+ 1 row in set, 1 warning (0.00 sec) (dg1)root@127.0.0.1 [mytest]>
总结：
1.整数类型，浮点数类型，时间类型的索引长度
NOT NULL=字段本身的字段长度
NULL=字段本身的字段长度+1，因为需要有是否为空的标记，这个标记需要占用1个字节
2.字符类型
varchr(n)变长字段且允许NULL = n * ( utf8=3,gbk=2,latin1=1)+1(NULL)+2 varchr(n)变长字段且不允许NULL = n * ( utf8=3,gbk=2,latin1=1)+2 char(n)固定字段且允许NULL = n * ( utf8=3,gbk=2,latin1=1)+1(NULL) char(n)固定字段且允许NULL = n * ( utf8=3,gbk=2,latin1=1) 变长字段需要额外的 本文系统来源：http://www.cnblogs.com/LMySQL/p/4525867.html
内容总结
以上是互联网集市为您收集整理的MySQL执行计划explain的key_len解析全部内容，希望文章能够帮你解决MySQL执行计划explain的key_len解析所遇到的程序开发问题。 如果觉得互联网集市技术教程内容还不错，欢迎将互联网集市网站推荐给程序员好友。
内容备注
版权声明：本文内容由互联网用户自发贡献，该文观点与技术仅代表作者本人。本站仅提供信息存储空间服务，不拥有所有权，不承担相关法律责任。如发现本站有涉嫌侵权/违法违规的内容， 请发送邮件至 gblab@vip.qq.com 举报，一经查实，本站将立刻删除。
内容手机端
扫描二维码推送至手机访问。
来源：【匿名】