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