E.9.2. The Maximum Number of Columns Per Table

There is a hard limit of 4096 columns per table, but the effective maximum may be less for a given table. The exact limit depends on several interacting factors, listed in the following discussion.

  • 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 permitted. NULL columns require additional space in the row to record whether their values are NULL.

    For MyISAM tables, each NULL column takes one bit extra, 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 for InnoDB tables, for which storage size is no different for NULL columns than for NOT NULL columns.

    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
    
  • Each table has an .frm file that contains the table definition. The server uses the following expression to check some of the table information stored in the file against an upper limit of 64KB:

    if (info_length+(ulong) create_fields.elements*FCOMP+288+
        n_length+int_length+com_length > 65535L || int_count > 255)
    

    The portion of the information stored in the .frm file that is checked against the expression cannot grow beyond the 64KB limit, so if the table definition reaches this size, no more columns can be added.

    The relevant factors in the expression are:

    • info_length is space needed for “screens.” This is related to MySQL's Unireg heritage.

    • create_fields.elements is the number of columns.

    • FCOMP is 17.

    • n_length is the total length of all column names, including one byte per name as a separator.

    • int_length is related to the list of values for ENUM and SET columns.

    • com_length is the total length of column and table comments.

    Thus, using long column names can reduce the maximum number of columns, as can the inclusion of ENUM or SET columns, or use of column or table comments.

  • Individual storage engines might impose additional restrictions that limit table column count. Examples:

    • InnoDB permits no more than 1000 columns.

    • InnoDB restricts row size to something less than half a database page (approximately 8000 bytes), not including VARBINARY, VARCHAR, BLOB, or TEXT columns.

    • Different InnoDB storage formats (COMPRESSED, REDUNDANT) use different amounts of page header and trailer data, which affects the amount of storage available for rows.

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