MyISAM is the default table type in MySQL Version 3.23. It's based on the ISAM code and
has a lot of useful extensions.
The index is stored in a le with the .MYI (MYIndex) extension, and the data is stored
in a le with the .MYD (MYData) extension. You can check/repair MyISAM tables with the
myisamchk utility.
The following is new in MyISAM:
If mysqld is started with --myisam-recover, MyISAM tables will automaticly be repaired on open if the table wasn't closed properly.
You can INSERT new rows in a table without deleted rows, while other threads are
reading from the table.
Support for big les 63-bit on lesystems/operating systems that support big les.
All data is stored with the low byte rst. This makes the data machine and OS
independent. The only requirement is that the machine uses two's-complement signed
integers (as every machine for the last 20 years has) and IEEE
oating-point format
also totally dominant among mainstream machines. The only area of machines that
may not support binary compatibility are embedded systems because they sometimes
have peculiar processors.
There is no big speed penalty in storing data low byte rst; The bytes in a table row
is normally unaligned and it doesn't take that much more power to read an unaligned
byte in order than in reverse order. The actual fetch-column-value code is also not
time critical compared to other code.
All number keys are stored with high byte rst to give better index compression.
Internal handling of one AUTO_INCREMENT column. MyISAM will automatically update
this on INSERT/UPDATE. The AUTO_INCREMENT value can be reset with myisamchk.
This will make AUTO_INCREMENT columns faster (at least 10 %) and old numbers will
not be reused as with the old ISAM. Note that when an AUTO_INCREMENT is de ned on
the end of a multi-part-key the old behavior is still present.
When inserted in sorted order (as when you are using an AUTO_INCREMENT column) the
key tree will be split so that the high node only contains one key. This will improve
the space utilization in the key tree.
BLOB and TEXT columns can be indexed.
NULL values are allowed in indexed columns. This takes 0-1 bytes/key.
Maximum key length is now 500 bytes by default. In cases of keys longer than 250
bytes, a bigger key block size than the default of 1024 bytes is used for this key.
Maximum number of keys/table enlarged to 32 as default. This can be enlarged to 64
without having to recompile myisamchk.
There is a ag in the MyISAM le that indicates whether or not the table was closed
correctly. This will soon be used for automatic repair in the MySQL server.
myisamchk will mark tables as checked if one runs it with --update-state. myisamchk
--fast will only check those tables that don't have this mark.
myisamchk -a stores statistics for key parts (and not only for whole keys as in ISAM).
Dynamic size rows will now be much less fragmented when mixing deletes with updates
and inserts. This is done by automatically combining adjacent deleted blocks and by
extending blocks if the next block is deleted.
myisampack can pack BLOB and VARCHAR columns.
has a lot of useful extensions.
The index is stored in a le with the .MYI (MYIndex) extension, and the data is stored
in a le with the .MYD (MYData) extension. You can check/repair MyISAM tables with the
myisamchk utility.
The following is new in MyISAM:
If mysqld is started with --myisam-recover, MyISAM tables will automaticly be repaired on open if the table wasn't closed properly.
You can INSERT new rows in a table without deleted rows, while other threads are
reading from the table.
Support for big les 63-bit on lesystems/operating systems that support big les.
All data is stored with the low byte rst. This makes the data machine and OS
independent. The only requirement is that the machine uses two's-complement signed
integers (as every machine for the last 20 years has) and IEEE
oating-point format
also totally dominant among mainstream machines. The only area of machines that
may not support binary compatibility are embedded systems because they sometimes
have peculiar processors.
There is no big speed penalty in storing data low byte rst; The bytes in a table row
is normally unaligned and it doesn't take that much more power to read an unaligned
byte in order than in reverse order. The actual fetch-column-value code is also not
time critical compared to other code.
All number keys are stored with high byte rst to give better index compression.
Internal handling of one AUTO_INCREMENT column. MyISAM will automatically update
this on INSERT/UPDATE. The AUTO_INCREMENT value can be reset with myisamchk.
This will make AUTO_INCREMENT columns faster (at least 10 %) and old numbers will
not be reused as with the old ISAM. Note that when an AUTO_INCREMENT is de ned on
the end of a multi-part-key the old behavior is still present.
When inserted in sorted order (as when you are using an AUTO_INCREMENT column) the
key tree will be split so that the high node only contains one key. This will improve
the space utilization in the key tree.
BLOB and TEXT columns can be indexed.
NULL values are allowed in indexed columns. This takes 0-1 bytes/key.
Maximum key length is now 500 bytes by default. In cases of keys longer than 250
bytes, a bigger key block size than the default of 1024 bytes is used for this key.
Maximum number of keys/table enlarged to 32 as default. This can be enlarged to 64
without having to recompile myisamchk.
There is a ag in the MyISAM le that indicates whether or not the table was closed
correctly. This will soon be used for automatic repair in the MySQL server.
myisamchk will mark tables as checked if one runs it with --update-state. myisamchk
--fast will only check those tables that don't have this mark.
myisamchk -a stores statistics for key parts (and not only for whole keys as in ISAM).
Dynamic size rows will now be much less fragmented when mixing deletes with updates
and inserts. This is done by automatically combining adjacent deleted blocks and by
extending blocks if the next block is deleted.
myisampack can pack BLOB and VARCHAR columns.
