12.6 Limits Imposed by .frm File Structure

As described previously, 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. In this context, “int” does not mean “integer.” It means “interval,” a term that refers collectively to ENUM and SET columns.

• int_count is the number of unique ENUM and SET definitions.

• com_length is the total length of column comments.

The expression just described has several implications for permitted table definitions:

• Using long column names can reduce the maximum number of columns, as can the inclusion of ENUM or SET columns, or use of column comments.

• A table can have no more than 255 unique ENUM and SET definitions. Columns with identical element lists are considered the same against this limt. For example, if a table contains these two columns, they count as one (not two) toward this limit because the definitions are identical:

  e1 ENUM('a','b','c')
  e2 ENUM('a','b','c')

• The sum of the length of element names in the unique ENUM and SET definitions counts toward the 64KB limit, so although the theoretical limit on number of elements in a given ENUM column is 65,535, the practical limit is less than 3000.