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_lengthis space needed for “screens.” This is related to MySQL's Unireg heritage.
create_fields.elementsis the number of columns.
n_lengthis the total length of all column names, including one byte per name as a separator.
com_lengthis the total length of column comments.
The expression just described has several implications for permitted table definitions:
A table can have no more than 255 unique
SETdefinitions. 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
SETdefinitions counts toward the 64KB limit, so although the theoretical limit on number of elements in a given
ENUMcolumn is 65,535, the practical limit is less than 3000.