### C.7.5 Limits Imposed by .frm File Structure

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.