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
create_fields.elements is the number of
FCOMP is 17.
n_length is the total length of all
column names, including one byte per name as a separator.
com_length is the total length of column
The expression just described has several implications for permitted table definitions:
A table can have no more than 255 unique
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
SET definitions counts toward
the 64KB limit, so although the theoretical limit on number
of elements in a given
column is 65,535, the practical limit is less than 3000.