Oracle 5.0, Справочное руководство

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Server System Variables
458
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
join_buffer_size
[457] to get a faster full join when
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
join_buffer_size
[457] is 4GB.
For additional information about join buffering, see Section 8.3.1.8, “Nested-Loop Join Algorithms” .
•
keep_files_on_create
[458]
Version Introduced 5.0.48
Command-Line Format
--keep_files_on_create=#
Option-File Format
keep_files_on_create
Option Sets Variable Yes,
keep_files_on_create
[458]
Variable Name
keep_files_on_create
Variable Scope Global, Session
Dynamic Variable Yes
Permitted Values
Type
boolean
Default
OFF
If a
MyISAM
table is created with no
DATA DIRECTORY
option, the
.MYD
file is created in the
database directory. By default, if
MyISAM
finds an existing
.MYD
file in this case, it overwrites it. The
same applies to
.MYI
files for tables created with no
INDEX DIRECTORY
option. To suppress this
behavior, set the
keep_files_on_create
[458] variable to
ON
(1), in which case
MyISAM
will not
overwrite existing files and returns an error instead. The default value is
OFF
(0).
If a
MyISAM
table is created with a
DATA DIRECTORY
or
INDEX DIRECTORY
option and an existing
.MYD
or
.MYI
file is found, MyISAM always returns an error. It will not overwrite a file in the specified
directory.
This variable was added in MySQL 5.0.48.
•
key_buffer_size
[458]
Command-Line Format
--key_buffer_size=#
Option-File Format
key_buffer_size
Option Sets Variable Yes,
key_buffer_size
[458]
Variable Name
key_buffer_size
Variable Scope Global
Dynamic Variable Yes
Index blocks for
MyISAM
tables are buffered and are shared by all threads.
key_buffer_size
[458] is the size of the buffer used for index blocks. The key buffer is also
known as the key cache.