Oracle 5.0, Справочное руководство

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Windows Platform Limitations
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if (info(ulong) create_fields.elements*FCOMP+288+
nintcom_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.
E.7.6. Windows Platform Limitations
The following limitations apply to use of MySQL on the Windows platform:
• Number of file descriptors
The number of open file descriptors on Windows is limited to a maximum of 2048, which may limit
the ability to open a large number of tables simultaneously. This limit is due not to Windows but
to C runtime library compatibility functions used to open files on Windows that use the POSIX
compatibility layer.
This limitation will also cause problems if you try to set
open_files_limit
to a value greater than
the 2048 file limit.
• Process memory
On Windows 32-bit platforms it is not possible by default to use more than 2GB of RAM within a
single process, including MySQL. This is because the physical address limit on Windows 32-bit
is 4GB and the default setting within Windows is to split the virtual address space between kernel
(2GB) and user/applications (2GB).
Some versions of Windows have a boot time setting to enable larger applications by reducing the
kernel application. Alternatively, to use more than 2GB, use a 64-bit version of Windows.
• File system aliases