Maximum Number of Columns Per Table – Mysql

Posted on August 10, 2013 by Suresh Kamrushi in Mysql

Maximum we can have 4096 columns per table, but exact limit depends on several factors, listed below:

1) 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.

2) 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:

  1. info_length is space needed for “screens.” This is related to MySQL’s Unireg heritage.
  2. create_fields.elements is the number of columns.
  3. FCOMP is 17.
  4. n_length is the total length of all column names, including one byte per name as a separator.
  5. int_length is related to the list of values for ENUM and SET columns.
  6. 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 SETcolumns, or use of column or table comments.

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

    a) InnoDB permits no more than 1000 columns.

b) InnoDB restricts row size to something less than half a database page (approximately 8000 bytes), not includingVARBINARYVARCHARBLOB, or TEXT columns.

  Comments or questions are welcome  
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