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(mysql.info)myisampack
myisampack, The MySQL Compressed Read-only Table Generator
----------------------------------------------------------
`myisampack' is used to compress MyISAM tables, and `pack_isam' is used
to compress ISAM tables. Because ISAM tables are deprecated, we will
only discuss `myisampack' here, but everything said about `myisampack'
should also be true for `pack_isam'.
`myisampack' works by compressing each column in the table separately.
The information needed to decompress columns is read into memory when
the table is opened. This results in much better performance when
accessing individual records, because you only have to uncompress
exactly one record, not a much larger disk block as when using Stacker
on MS-DOS. Usually, `myisampack' packs the data file 40%-70%.
MySQL uses memory mapping (`mmap()') on compressed tables and falls
back to normal read/write file usage if `mmap()' doesn't work.
There are currently two limitations with `myisampack':
* After packing, the table is read-only.
* `myisampack' can also pack `BLOB' or `TEXT' columns. The older
`pack_isam' could not do this.
Fixing these limitations is on our TODO list but with low priority.
`myisampack' is invoked like this:
shell> myisampack [options] filename ...
Each filename should be the name of an index (`.MYI') file. If you are
not in the database directory, you should specify the pathname to the
file. It is permissible to omit the `.MYI' extension.
`myisampack' supports the following options:
`-b, --backup'
Make a backup of the table as `tbl_name.OLD'.
`-#, --debug=debug_options'
Output debug log. The `debug_options' string often is
`'d:t:o,filename''.
`-f, --force'
Force packing of the table even if it becomes bigger or if the
temporary file exists. `myisampack' creates a temporary file
named `tbl_name.TMD' while it compresses the table. If you kill
`myisampack', the `.TMD' file may not be deleted. Normally,
`myisampack' exits with an error if it finds that `tbl_name.TMD'
exists. With `--force', `myisampack' packs the table anyway.
`-?, --help'
Display a help message and exit.
`-j big_tbl_name, --join=big_tbl_name'
Join all tables named on the command line into a single table
`big_tbl_name'. All tables that are to be combined MUST be
identical (same column names and types, same indexes, etc.).
`-p #, --packlength=#'
Specify the record length storage size, in bytes. The value
should be 1, 2, or 3. (`myisampack' stores all rows with length
pointers of 1, 2, or 3 bytes. In most normal cases, `myisampack'
can determine the right length value before it begins packing the
file, but it may notice during the packing process that it could
have used a shorter length. In this case, `myisampack' will print
a note that the next time you pack the same file, you could use a
shorter record length.)
`-s, --silent'
Silent mode. Write output only when errors occur.
`-t, --test'
Don't actually pack table, just test packing it.
`-T dir_name, --tmp_dir=dir_name'
Use the named directory as the location in which to write the
temporary table.
`-v, --verbose'
Verbose mode. Write information about progress and packing result.
`-V, --version'
Display version information and exit.
`-w, --wait'
Wait and retry if table is in use. If the `mysqld' server was
invoked with the `--skip-locking' option, it is not a good idea to
invoke `myisampack' if the table might be updated during the
packing process.
The sequence of commands shown below illustrates a typical table
compression session:
shell> ls -l station.*
-rw-rw-r-- 1 monty my 994128 Apr 17 19:00 station.MYD
-rw-rw-r-- 1 monty my 53248 Apr 17 19:00 station.MYI
-rw-rw-r-- 1 monty my 5767 Apr 17 19:00 station.frm
shell> myisamchk -dvv station
MyISAM file: station
Isam-version: 2
Creation time: 1996-03-13 10:08:58
Recover time: 1997-02-02 3:06:43
Data records: 1192 Deleted blocks: 0
Datafile: Parts: 1192 Deleted data: 0
Datafile pointer (bytes): 2 Keyfile pointer (bytes): 2
Max datafile length: 54657023 Max keyfile length: 33554431
Recordlength: 834
Record format: Fixed length
table description:
Key Start Len Index Type Root Blocksize Rec/key
1 2 4 unique unsigned long 1024 1024 1
2 32 30 multip. text 10240 1024 1
Field Start Length Type
1 1 1
2 2 4
3 6 4
4 10 1
5 11 20
6 31 1
7 32 30
8 62 35
9 97 35
10 132 35
11 167 4
12 171 16
13 187 35
14 222 4
15 226 16
16 242 20
17 262 20
18 282 20
19 302 30
20 332 4
21 336 4
22 340 1
23 341 8
24 349 8
25 357 8
26 365 2
27 367 2
28 369 4
29 373 4
30 377 1
31 378 2
32 380 8
33 388 4
34 392 4
35 396 4
36 400 4
37 404 1
38 405 4
39 409 4
40 413 4
41 417 4
42 421 4
43 425 4
44 429 20
45 449 30
46 479 1
47 480 1
48 481 79
49 560 79
50 639 79
51 718 79
52 797 8
53 805 1
54 806 1
55 807 20
56 827 4
57 831 4
shell> myisampack station.MYI
Compressing station.MYI: (1192 records)
- Calculating statistics
normal: 20 empty-space: 16 empty-zero: 12 empty-fill: 11
pre-space: 0 end-space: 12 table-lookups: 5 zero: 7
Original trees: 57 After join: 17
- Compressing file
87.14%
shell> ls -l station.*
-rw-rw-r-- 1 monty my 127874 Apr 17 19:00 station.MYD
-rw-rw-r-- 1 monty my 55296 Apr 17 19:04 station.MYI
-rw-rw-r-- 1 monty my 5767 Apr 17 19:00 station.frm
shell> myisamchk -dvv station
MyISAM file: station
Isam-version: 2
Creation time: 1996-03-13 10:08:58
Recover time: 1997-04-17 19:04:26
Data records: 1192 Deleted blocks: 0
Datafile: Parts: 1192 Deleted data: 0
Datafilepointer (bytes): 3 Keyfile pointer (bytes): 1
Max datafile length: 16777215 Max keyfile length: 131071
Recordlength: 834
Record format: Compressed
table description:
Key Start Len Index Type Root Blocksize Rec/key
1 2 4 unique unsigned long 10240 1024 1
2 32 30 multip. text 54272 1024 1
Field Start Length Type Huff tree Bits
1 1 1 constant 1 0
2 2 4 zerofill(1) 2 9
3 6 4 no zeros, zerofill(1) 2 9
4 10 1 3 9
5 11 20 table-lookup 4 0
6 31 1 3 9
7 32 30 no endspace, not_always 5 9
8 62 35 no endspace, not_always, no empty 6 9
9 97 35 no empty 7 9
10 132 35 no endspace, not_always, no empty 6 9
11 167 4 zerofill(1) 2 9
12 171 16 no endspace, not_always, no empty 5 9
13 187 35 no endspace, not_always, no empty 6 9
14 222 4 zerofill(1) 2 9
15 226 16 no endspace, not_always, no empty 5 9
16 242 20 no endspace, not_always 8 9
17 262 20 no endspace, no empty 8 9
18 282 20 no endspace, no empty 5 9
19 302 30 no endspace, no empty 6 9
20 332 4 always zero 2 9
21 336 4 always zero 2 9
22 340 1 3 9
23 341 8 table-lookup 9 0
24 349 8 table-lookup 10 0
25 357 8 always zero 2 9
26 365 2 2 9
27 367 2 no zeros, zerofill(1) 2 9
28 369 4 no zeros, zerofill(1) 2 9
29 373 4 table-lookup 11 0
30 377 1 3 9
31 378 2 no zeros, zerofill(1) 2 9
32 380 8 no zeros 2 9
33 388 4 always zero 2 9
34 392 4 table-lookup 12 0
35 396 4 no zeros, zerofill(1) 13 9
36 400 4 no zeros, zerofill(1) 2 9
37 404 1 2 9
38 405 4 no zeros 2 9
39 409 4 always zero 2 9
40 413 4 no zeros 2 9
41 417 4 always zero 2 9
42 421 4 no zeros 2 9
43 425 4 always zero 2 9
44 429 20 no empty 3 9
45 449 30 no empty 3 9
46 479 1 14 4
47 480 1 14 4
48 481 79 no endspace, no empty 15 9
49 560 79 no empty 2 9
50 639 79 no empty 2 9
51 718 79 no endspace 16 9
52 797 8 no empty 2 9
53 805 1 17 1
54 806 1 3 9
55 807 20 no empty 3 9
56 827 4 no zeros, zerofill(2) 2 9
57 831 4 no zeros, zerofill(1) 2 9
The information printed by `myisampack' is described below:
`normal'
The number of columns for which no extra packing is used.
`empty-space'
The number of columns containing values that are only spaces;
these will occupy 1 bit.
`empty-zero'
The number of columns containing values that are only binary 0's;
these will occupy 1 bit.
`empty-fill'
The number of integer columns that don't occupy the full byte
range of their type; these are changed to a smaller type (for
example, an `INTEGER' column may be changed to `MEDIUMINT').
`pre-space'
The number of decimal columns that are stored with leading spaces.
In this case, each value will contain a count for the number of
leading spaces.
`end-space'
The number of columns that have a lot of trailing spaces. In this
case, each value will contain a count for the number of trailing
spaces.
`table-lookup'
The column had only a small number of different values, which were
converted to an `ENUM' before Huffman compression.
`zero'
The number of columns for which all values are zero.
`Original trees'
The initial number of Huffman trees.
`After join'
The number of distinct Huffman trees left after joining trees to
save some header space.
After a table has been compressed, `myisamchk -dvv' prints additional
information about each field:
`Type'
The field type may contain the following descriptors:
`constant'
All rows have the same value.
`no endspace'
Don't store endspace.
`no endspace, not_always'
Don't store endspace and don't do end space compression for
all values.
`no endspace, no empty'
Don't store endspace. Don't store empty values.
`table-lookup'
The column was converted to an `ENUM'.
`zerofill(n)'
The most significant `n' bytes in the value are always 0 and
are not stored.
`no zeros'
Don't store zeros.
`always zero'
0 values are stored in 1 bit.
`Huff tree'
The Huffman tree associated with the field.
`Bits'
The number of bits used in the Huffman tree.
After you have run `pack_isam'/`myisampack' you must run
`isamchk'/`myisamchk' to re-create the index. At this time you can
also sort the index blocks and create statistics needed for the MySQL
optimizer to work more efficiently:
myisamchk -rq --analyze --sort-index table_name.MYI
isamchk -rq --analyze --sort-index table_name.ISM
After you have installed the packed table into the MySQL database
directory you should do `mysqladmin flush-tables' to force `mysqld' to
start using the new table.
If you want to unpack a packed table, you can do this with the
`--unpack' option to `isamchk' or `myisamchk'.
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