mysql图⽚路径varchar⼤⼩_mysqlvarchar长度问题
Every table has a maximum row size of 65,535 bytes. This maximum applies to all storage engines, but a given engine might have additional constraints that result in a lower effective maximum row size.
The maximum row size constrains the number of columns because the total width of all columns cannot exceed this size. For example, utf8 characters require up to three bytes per character, so for a CHAR(255) CHARACTER SET utf8 column, the server must allocate 255 × 3 = 765 bytes per value. Consequently, a table cannot contain more than 65,535 / 765 = 85 such columns.
Storage for variable-length columns includes length bytes, which are assessed against the row size. For example, a VARCHAR(255) CHARACTER SET utf8 column takes two bytes to store the length of the value, so each value can take up to 767 bytes.
BLOB and TEXT columns count from one to four plus eight bytes each toward the row-size limit because their contents are stored separately.
Declaring columns NULL can reduce the maximum number of columns allowed. NULL columns require additional space in the row to record whether their values are NULL.
ForMyISAMtables, eachNULLcolumn takes one bit extra(注意是bit), rounded up to the nearest byte. The maximum row
length in bytes can be calculated as follows:row length = 1
+ (sum of column lengths)
+ (number of NULL columns + delete_flag + 7)/8
+ (number of variable-length columns)
delete_flag is 1 for tables with static row format. Static tables use a bit in the row record for a flag that indicates whether
the row has been deleted. delete_flag is 0 for dynamic tables because the flag is stored in the dynamic row header.
These calculations do not apply forInnoDBtables, for which storage size is no different forNULLcolumns than forNOT NULLcolumns.
The following statement to create table t1 succeeds because the columns require 32,765 + 2 bytes and 32,766 + 2 bytes, which falls within the maximum row size of 65,535 bytes:mysql> CREATE TABLE t1
-> (c1 VARCHAR(32765) NOT NULL, c2 VARCHAR(32766) NOT NULL);
Query OK, 0 rows affected (0.01 sec)
The following statement to create table t2 fails because the columns are NULL and require additional space that causes the row size to exceed 65,535 bytes:mysql> CREATE TABLE t2
-> (c1 VARCHAR(32765) NULL, c2 VARCHAR(32766) NULL);
ERROR 1118 (42000): Row size too large. The maximum row size for the
used table type, not counting BLOBs, is 65535. You have to change some
columns to TEXT or BLOBs
>varchar2最大长度

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