You can not select more than 25 topics
Topics must start with a letter or number, can include dashes ('-') and can be up to 35 characters long.
66 lines
3.4 KiB
66 lines
3.4 KiB
New features compared to NISAM:
|
|
|
|
- All file positions have type my_off_t; This enables one to use big
|
|
files (2^63 byte) by defining my_off_t to be longlong on OS that supports
|
|
big files.
|
|
- When creating a table, one can now specify the maximum data file length.
|
|
This will be used to calculate the length of row pointers.
|
|
- All key segments have their own language definition.
|
|
- Some changes to support more types:
|
|
The biggest change is that the interface allows MY_ISAM will support
|
|
variable length integer types. (Only the interface is implemented)
|
|
- All data is stored with low byte first; This makes the data machine
|
|
independent.
|
|
- All number keys are stored with high byte first to give better packing.
|
|
- Support for a true VARCHAR type; A VARCHAR column starts with a length
|
|
stored on 2 bytes.
|
|
- Tables with VARCHAR may have fixed or dynamic record length.
|
|
- There are now 2 different ways to pack keys:
|
|
- If the first key part is a space stripped CHAR, a VARCHAR or a BLOB the
|
|
'packed' method is used. This only prefix-compresses the first
|
|
key part.
|
|
- In other cases prefix packing is used (This also includes the record
|
|
pointer into the prefix packing). A key may in the best case be
|
|
packed on 2 bytes.
|
|
- VARCHAR and CHAR may be up to 65K
|
|
- Index on BLOB and VARCHAR.
|
|
- One can now have NULL in an index. This takes 0-1 bytes / key.
|
|
- MYISAM will allow one to specify one AUTO_INCREMENT column; MYISAM will
|
|
automaticly update this on INSERT/UPDATE. The AUTO_INCREMENT value can be
|
|
reset with myisamchk.
|
|
- Max key length will be 500 by default; In cases of longer keys than 250,
|
|
a bigger key block size than the default of 1024 byes is used for this key.
|
|
- Max number of keys enlarged to 32 as default. This can be enlarged to 64
|
|
without having to recompile myisamchk.
|
|
- There is a flag in the MYISAM header that tells if the index file (.MYI)
|
|
was closed correctly.
|
|
- myisamchk will now mark tables as checked. 'myisamchk --fast' will only
|
|
check those tables that doesn't have this mark.
|
|
- 'myisamchk -a' stores statistic for key parts (and not only for whole keys
|
|
as in NISAM).
|
|
- Dynamic size rows will now be much less fragmented when mixing deletes with
|
|
update and insert. This is done by automaticly combine adjacent deleted
|
|
blocks and by extending blocks if the next block is deleted.
|
|
- For dynamic size rows, the delete link contains a pointer to itself
|
|
(to make repairs easier).
|
|
- myisampack (called pack_isam in NISAM) can pack BLOB and VARCHAR
|
|
columns.
|
|
- One can now disable any key from update; In NISAM one could only disable
|
|
the last x keys.
|
|
- One can have a UNIQUE constraint on anything (including BLOBS).
|
|
This is implemented by a key that contains a hashed number of the whole
|
|
record and before inserting a new record, MyISAM will check all records
|
|
with the same hash for dupplicates.
|
|
- When creating the table, one can define that MyISAM should maintain
|
|
a CRC for the whole table (to make isamchk even better). In the case of
|
|
dynamic size rows, one will in this case get 1 byte checksum for each row.
|
|
(This is a great help for debugging, but it can also be used to keep
|
|
MyISAM table 'extra' safe.
|
|
- Temporary tables will not write not flushed keys to file on close and
|
|
not wait on 'disk full' conditions.
|
|
|
|
Interface changes compared to NISAM:
|
|
|
|
- mi_create()
|
|
- keyinfo->seg must be allocated explicitely.
|
|
- One must put number of key segments in keyinfo
|
|
|