mysql 占用的内存大小-白红宇

mysql 占用的内存大小

阅读量：5338 次

发布时间：2019-06-15

本文共 4898 字，大约阅读时间需要 16 分钟。

1、mysql执行查询计划，key_len表示索引使用的字节数，这个字节数和三个条件有关。

mysql> create table t1(v1 char(10));

Query OK, 0 rows affected

mysql> create index index_v1 on t1(v1);

Query OK, 0 rows affected

Records: 0 Duplicates: 0 Warnings: 0

2、是否允许为null，允许为null 会多一个字节，标识取值是否为NULL。如下：

mysql> alter table t1 modify column v1 char(10) not null;

Query OK, 0 rows affected

Records: 0 Duplicates: 0 Warnings: 0

mysql> desc select * from t1;

+----+-------------+-------+-------+---------------+----------+---------+------+------+-------------+

+----+-------------+-------+-------+---------------+----------+---------+------+------+-------------+

+----+-------------+-------+-------+---------------+----------+---------+------+------+-------------+

1 row in set

mysql> alter table t1 modify column v1 char(10) null;

Query OK, 0 rows affected

Records: 0 Duplicates: 0 Warnings: 0

mysql> desc select * from t1;

+----+-------------+-------+-------+---------------+----------+---------+------+------+-------------+

+----+-------------+-------+-------+---------------+----------+---------+------+------+-------------+

+----+-------------+-------+-------+---------------+----------+---------+------+------+-------------+

1 row in set

3、是否为变长，变长多出两个字节表示长度，如下：

mysql> alter table t1 modify column v1 varchar(10) null;

Query OK, 0 rows affected

Records: 0 Duplicates: 0 Warnings: 0

mysql> desc select * from t1;

+----+-------------+-------+-------+---------------+----------+---------+------+------+-------------+

+----+-------------+-------+-------+---------------+----------+---------+------+------+-------------+

+----+-------------+-------+-------+---------------+----------+---------+------+------+-------------+

1 row in set

mysql> alter table t1 modify column v1 char(10) null;

Query OK, 0 rows affected

Records: 0 Duplicates: 0 Warnings: 0

mysql> desc select * from t1;

+----+-------------+-------+-------+---------------+----------+---------+------+------+-------------+

+----+-------------+-------+-------+---------------+----------+---------+------+------+-------------+

+----+-------------+-------+-------+---------------+----------+---------+------+------+-------------+

1 row in set

4、字符类型，varchar(n) 中的n是字符个数，不是占用多少个字节的内存。对于不同的编码，同样一个字符占用的内存不一样。latin1 一个字符占用1个字节，gb2312 一个字符占用2个字节，utf8 一个字符占用3个字节。

mysql> alter table t1 modify column v1 char(10) charset latin1;

Query OK, 0 rows affected

Records: 0 Duplicates: 0 Warnings: 0

mysql> desc select * from t1;

+----+-------------+-------+-------+---------------+----------+---------+------+------+-------------+

+----+-------------+-------+-------+---------------+----------+---------+------+------+-------------+

+----+-------------+-------+-------+---------------+----------+---------+------+------+-------------+

1 row in set

mysql> alter table t1 modify column v1 char(10) charset gb2312;

Query OK, 0 rows affected

Records: 0 Duplicates: 0 Warnings: 0

mysql> desc select * from t1;

+----+-------------+-------+-------+---------------+----------+---------+------+------+-------------+

+----+-------------+-------+-------+---------------+----------+---------+------+------+-------------+

+----+-------------+-------+-------+---------------+----------+---------+------+------+-------------+

1 row in set

mysql> alter table t1 modify column v1 char(10) charset utf8;

Query OK, 0 rows affected

Records: 0 Duplicates: 0 Warnings: 0

mysql> desc select * from t1;

+----+-------------+-------+-------+---------------+----------+---------+------+------+-------------+

+----+-------------+-------+-------+---------------+----------+---------+------+------+-------------+

+----+-------------+-------+-------+---------------+----------+---------+------+------+-------------+

1 row in set

转载于:https://www.cnblogs.com/nzbbody/p/4605673.html

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