MYISAMPACK(1)

🌸 NAME 🌸

so bestie, this is myisampack — the utility from the MYISAMPACK(1) manual — and she does exactly one thing: generate compressed, read-only MyISAM tables. periodt. 💖

✨ SYNOPSIS ✨

here's the vibe check on how you call her:

myisampack [options] file_name ...

💖 DESCRIPTION 💖

okay so the myisampack utility compresses MyISAM tables. she works by compressing each column in the table separately. usually, myisampack packs the data file 40%–70%. that's a glow-up fr fr. ✨

when the table is used later, the server reads into memory the information needed to decompress columns. this results in much better performance when accessing individual rows, because you only have to uncompress exactly one row. the way that works is lowkey genius. 🧚

MariaDB uses mmap() when possible to perform memory mapping on compressed tables. if mmap() does not work, MariaDB falls back to normal read/write file operations. no cap.

please note the following:

• if the mariadbd server was invoked with external locking disabled, it is not a good idea to invoke myisampack if the table might be updated by the server during the packing process. it is safest to compress tables with the server stopped. 💀
• after packing a table, it becomes read only. this is generally intended (such as when accessing packed tables on a CD). so don't come for her, she's doing what she's supposed to. 🎀

invoke myisampack like this:

shell> myisampack [options] file_name ...

each file name argument should be the name of an index (.MYI) file. if you are not in the database directory, you should specify the path name to the file. it is permissible to omit the .MYI extension. iykyk.

after you compress a table with myisampack, you should use myisamchk -rq to rebuild its indexes. myisamchk(1). don't skip this step bestie, it's giving necessary. 💅

myisampack supports the following options. it also reads option files and supports the options for processing them described at Section 4.2.3.3.1, “Command-Line Options that Affect Option-File Handling”. so many options, so little time. 😭

• --help, -? — display a help message and exit. when you're lost, she's got you. 🫶
• --backup, -b — make a backup of each table's data file using the name tbl_name.OLD. safety first, bestie. 💖
• --character-sets-dir=path — the directory where character sets are installed. see Section 9.5, “Character Set Configuration”. she needs to know where the vibes live. 🌈
• --debug[=debug_options], -# [debug_options] — write a debugging log. a typical debug_options string is 'd:t:o,file_name'. the default is 'd:t:o'. for when you need to see behind the scenes. 🔥
• --force, -f — produce a packed table even if it becomes larger than the original or if the intermediate file from an earlier invocation of myisampack exists. (myisampack creates an intermediate file named tbl_name.TMD in the database directory while it compresses the table. if you kill myisampack, the .TMD file might not be deleted.) normally, myisampack exits with an error if it finds that tbl_name.TMD exists. with --force, myisampack packs the table anyway. she's stubborn, but make it fashion. 💅
• --join=big_tbl_name, -j big_tbl_name — join all tables named on the command line into a single packed table big_tbl_name. all tables that are to be combined must have identical structure (same column names and types, same indexes, and so forth). big_tbl_name must not exist prior to the join operation. all source tables named on the command line to be merged into big_tbl_name must exist. the source tables are read for the join operation but not modified. the join operation does not create a .frm file for big_tbl_name, so after the join operation finishes, copy the .frm file from one of the source tables and name it big_tbl_name.frm. it's giving merge, but make it manual. 🧚
• --silent, -s — silent mode. write output only when errors occur. she's quiet, but she's listening. 🤫
• --test, -t — do not actually pack the table, just test packing it. a little dress rehearsal, no commitment. 🎀
• --tmpdir=path, -T path — use the named directory as the location where myisampack creates temporary files. keep it tidy. ✨
• --verbose, -v — verbose mode. write information about the progress of the packing operation and its result. she's telling you everything, and honestly? we love a transparent queen. 👑
• --version, -V — display version information and exit. know your era. 💖
• --wait, -w — wait and retry if the table is in use. if the mariadbd server was invoked with external locking disabled, it is not a good idea to invoke myisampack if the table might be updated by the server during the packing process. patience is a virtue, but also, maybe just stop the server. 💀

the following sequence of commands illustrates a typical table compression session:

...so like, here's the receipts bestie 💅 first we clock the files, then we peek inside station:

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

and then the actual glow-up ✨

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%
Remember to run myisamchk -rq on compressed tables
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
Datafile pointer (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

not Record format: Fixed length anymore bestie, it's Compressed now, and the Field section lowkey grew two whole new columns (Huff tree and Bits) like it went and got bangs 🙈

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myisampack spills the following kinds of information, and no we are not skipping a single one:

• normal — the number of columns where literally no extra packing got used. just vibing as-is.
• empty-space — how many columns hold values that are nothing but spaces. those take up one bit. slay.
• empty-zero — how many columns hold values that are only binary zeros. also one bit each. iconic efficiency.
• empty-fill — the number of integer columns that don't use the full byte range of their type, so they get swapped for a smaller type. like, a BIGINT column (eight bytes) can live as a TINYINT column (one byte) if every single value fits in the range from -128 to 127. glow-down for the win 💅
• pre-space — the number of decimal columns stored with leading spaces. in that case each value carries a count of how many leading spaces it has.
• end-space — the number of columns with a lot of trailing spaces. same deal, each value carries a count of the trailing spaces.
• table-lookup — this column only had a small number of different values, so it got converted to an ENUM before the Huffman compression happened.
• zero — the number of columns where every value is zero. all of them. zero.
• Original trees — the initial number of Huffman trees.
• After join — how many distinct Huffman trees are left after joining trees together to save some header space.

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once a table's been compressed, the Field lines that myisamchk -dvv shows you get extra intel about every column:

• Type — the data type. the value can contain any of these descriptors:
  • constant — every single row has the same value. consistent queen.
  • no endspace — do not store endspace.
  • no endspace, not_always — do not store endspace, and do not do endspace compression for all values.
  • no endspace, no empty — do not store endspace. do not store empty values.
  • table-lookup — the column got converted to an ENUM.
  • zerofill(N) — the most significant N bytes in the value are always 0 and are not stored.
  • no zeros — do not store zeros.
  • always zero — zero values get stored using one bit.
• Huff tree — the number of the Huffman tree that's associated with the column.
• Bits — how many bits are used in the Huffman tree.

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ok so mandatory homework time 🎀 after you run myisampack, you must run myisamchk to re-create any indexes. and while you're at it you can also sort the index blocks and create the statistics that the MariaDB optimizer needs to work more efficiently:

shell> myisamchk -rq --sort-index --analyze tbl_name.MYI

and once you've installed the packed table into the MariaDB database directory, you should run mariadb-admin flush-tables to force mariadbd to start using the new table. no cap.

and if you ever wanna unpack a packed table? use the --unpack option on myisamchk. that's it, that's the whole reversal 🦋

💖 COPYRIGHT 💖

okay so legal era, bestie, we gotta give credit where credit is due 🫶

Copyright 2007-2008 MySQL AB, 2008-2010 Sun Microsystems, Inc., 2010-2025 MariaDB Foundation

This documentation is free software; you can redistribute it and/or modify it only under the terms of the GNU General Public License as published by the Free Software Foundation; version 2 of the License. that's it, that's the deal 💅

This documentation is distributed in the hope that it will be useful, but WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU General Public License for more details. like fr fr, no promises were made, read the fine print ✨

You should have received a copy of the GNU General Public License along with the program; if not, write to the Free Software Foundation, Inc., 51 Franklin Street, Fifth Floor, Boston, MA 02110-1335 USA or see http://www.gnu.org/licenses/.

not me reading license text for fun 😭 but lowkey it matters, so here it is, all of it, untouched. no cap.

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🌈 SEE ALSO 🌈

For more information, please refer to the MariaDB Documentation, available online at https://mariadb.com/docs/ — that's the mothership, the main character, the place where all the lore lives 👑

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👑 AUTHOR 👑

MariaDB Foundation (http://www.mariadb.org/). icons. legends. the ones who wrote this whole thing so you could pack your MyISAM tables and move on with your day 🎀

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and that's a wrap on the whole page, bestie!! you made it to the end, which is giving dedication 💖 this was MariaDB 11.8, dated 5 March 2025, and yes the manual page is literally called MYISAMPACK(1) — go forth and compress responsibly, mwah 🦋✨