MySQL stores VARCHAR values as a 1-byte or 2-byte length prefix plus data. The length prefix indicates the number of bytes in the value. A VARCHAR column uses one length byte if values require no more than 255 bytes, two length bytes if values may require more than 255 bytes. VARCHAR ...
对于不同存储引擎,在存放VARCHAR(N)类型数据时采用不同的存储方式,对于Innodb存储引擎,使用额外来1-2byte空间来存放变长列的数据长度,因此数据使用的存储空间与N值无明显关系,N值过大也不会导致数据占用过多的磁盘空间。 当数据从存储引擎读取到MySQL内存中时,数据在存储引擎中存放方式和在内存中的存放方式不同,存...
下面梳理用于解析 MySQL 数据类型的 type 语义组,其中涉及的 symbol 及 symbol 之间的关系如下(图中绿色节点为字符串字面值涉及节点、蓝色节点为其他语义组、灰色节点为其他终结符): 语义组:type type 语义组用于解析 MySQL 中的数据类型。 官方文档:MySQL 参考手册 - Chapter 13 Data Types 返回值类型:PT_type ...
字符类型/string (character and byte) types 另外还包含两个没那么常用的大类: 特殊类型/spatial types JSON 继续之前,先来看一些单位上的约定和概念, M:根据具体不同的类型,其表示的意思不一样,见下方关于这个参数的讨论。 D 用于定点及浮点数,表示小数点后有多少位。最大可能取值为 30,但不应该超过 M-2...
We can see that names in aVARCHARcolumn type are stored in variable length. This saves disk space. Binary and varbinary BINARYandVARBINARYare binary byte data types. They contain byte strings rather than character strings. They have no character sets. Sorting and comparison are based on the num...
BIT data type Posted by:morteza ir Date: July 01, 2011 02:04AM Hi There is a table that have 4 bit fields. one of bit field is seprated from another 3 bit fields. I know the 4 bit fields use a share byte. But do order is nessecery? In this case, 4 bit fields use 1 or 2...
The CHAR BYTE data type is an alias for the BINARY data type. This is a compatibility feature. MySQL permits you to create a column of type CHAR(0). This is useful primarily when you must be compliant with old applications that depend on the existence of a column but that do not act...
TheCHAR BYTEdata type is an alias for theBINARYdata type. This is a compatibility feature. MySQL permits you to create a column of typeCHAR(0). This is useful primarily when you must be compliant with old applications that depend on the existence of a column but that do not actually use...
比如门牌号码101,201……这样很短的信息应该用char,因为varchar还要占个byte用于存储信息长度,本来打算节约存储的,结果得不偿失。 2、情况2:固定长度的。比如使用uuid作为主键,那用char应该更合适。因为他固定长度, varchar动态根据长度的特性就消失了,而且还要占个长度信息。 3、情况3:十分频繁改变的column。因为...