Oracle 5.0 Reference Manual page 478

The minimum size of the buffer that is used for plain index scans, range index scans, and joins
that do not use indexes and thus perform full table scans. Normally, the best way to get fast joins
is to add indexes. Increase the value of
adding indexes is not possible. One join buffer is allocated for each full join between two tables. For
a complex join between several tables for which indexes are not used, multiple join buffers might be
necessary. There is no gain from setting the buffer larger than required to hold each matching row,
and all joins allocate at least the minimum size, so use caution in setting this variable to a large value
globally. It is better to keep the global setting small and change to a larger setting only in sessions
that are doing large joins. Memory allocation time can cause substantial performance drops if the
global size is larger than needed by most queries that use it.
The maximum permissible setting for
For additional information about join buffering, see
•
keep_files_on_create
Version Introduced
Command-Line Format
Option-File Format
Option Sets Variable
Variable Name
Variable Scope
Dynamic Variable
If a table is created with no
MyISAM
database directory. By default, if
same applies to
.MYI
behavior, set the
keep_files_on_create
overwrite existing files and returns an error instead. The default value is
If a table is created with a
MyISAM
or file is found, MyISAM always returns an error. It will not overwrite a file in the specified
.MYD .MYI
directory.
This variable was added in MySQL 5.0.48.
•
key_buffer_size
Command-Line Format
Option-File Format
Option Sets Variable
Variable Name
Variable Scope
Dynamic Variable
Index blocks for
MyISAM
key_buffer_size
known as the key cache.
Server System Variables
join_buffer_size
join_buffer_size
[458]
5.0.48
--keep_files_on_create=#
keep_files_on_create
Yes,
keep_files_on_create
keep_files_on_create
Global, Session
Yes
Permitted Values
Type
boolean
Default
OFF
DATA DIRECTORY
finds an existing
MyISAM
files for tables created with no
DATA DIRECTORY
[458]
--key_buffer_size=#
key_buffer_size
Yes,
key_buffer_size
key_buffer_size
Global
Yes
tables are buffered and are shared by all threads.
[458] is the size of the buffer used for index blocks. The key buffer is also
458
[457]
[457] is 4GB.
Section 8.3.1.8, "Nested-Loop Join
[458]
option, the
.MYD
file in this case, it overwrites it. The
.MYD
INDEX DIRECTORY
[458] variable to (1), in which case
ON
or
INDEX DIRECTORY
[458]
to get a faster full join when
Algorithms".
file is created in the
option. To suppress this
MyISAM
(0).
OFF
option and an existing
will not