Provided by: mysql-server-5.7_5.7.42-0ubuntu0.18.04.1_amd64 
NAME
myisampack - generate compressed, read-only MyISAM tables
SYNOPSIS
myisampack [options] file_name ...
DESCRIPTION
The myisampack utility compresses MyISAM tables. myisampack works by compressing each column in the
table separately. Usually, myisampack packs the data file 40% to 70%.
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.
MySQL uses mmap() when possible to perform memory mapping on compressed tables. If mmap() does not work,
MySQL falls back to normal read/write file operations.
Please note the following:
• If the mysqld 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).
• myisampack does not support partitioned tables.
Invoke myisampack like this:
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.
After you compress a table with myisampack, use myisamchk -rq to rebuild its indexes. myisamchk(1).
myisampack supports the following options. It also reads option files and supports the options for
processing them described at Section 4.2.2.3, “Command-Line Options that Affect Option-File Handling”.
• --help, -? Display a help message and exit.
• --backup, -b Make a backup of each table's data file using the name tbl_name.OLD.
• --character-sets-dir=dir_name The directory where character sets are installed. See Section 10.15,
“Character Set Configuration”.
• --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.
This option is available only if MySQL was built using WITH_DEBUG. MySQL release binaries provided by
Oracle are not built using this option.
• --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.
• --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.
• --silent, -s Silent mode. Write output only when errors occur.
• --test, -t Do not actually pack the table, just test packing it.
• --tmpdir=dir_name, -T dir_name Use the named directory as the location where myisampack creates
temporary files.
• --verbose, -v Verbose mode. Write information about the progress of the packing operation and its
result.
• --version, -V Display version information and exit.
• --wait, -w Wait and retry if the table is in use. If the mysqld 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.
The following sequence of commands illustrates a typical table compression session:
$> ls -l station.*
-rw-rw-r-- 1 jones my 994128 Apr 17 19:00 station.MYD
-rw-rw-r-- 1 jones my 53248 Apr 17 19:00 station.MYI
-rw-rw-r-- 1 jones my 5767 Apr 17 19:00 station.frm
$> 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
$> 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
$> myisamchk -rq station
- check record delete-chain
- recovering (with sort) MyISAM-table 'station'
Data records: 1192
- Fixing index 1
- Fixing index 2
$> mysqladmin -uroot flush-tables
$> ls -l station.*
-rw-rw-r-- 1 jones my 127874 Apr 17 19:00 station.MYD
-rw-rw-r-- 1 jones my 55296 Apr 17 19:04 station.MYI
-rw-rw-r-- 1 jones my 5767 Apr 17 19:00 station.frm
$> 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
myisampack displays the following kinds of information:
• 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 occupy one bit.
• empty-zero
The number of columns containing values that are only binary zeros. These occupy one bit.
• empty-fill
The number of integer columns that do not occupy the full byte range of their type. These are changed
to a smaller type. For example, a BIGINT column (eight bytes) can be stored as a TINYINT column (one
byte) if all its values are in the range from -128 to 127.
• pre-space
The number of decimal columns that are stored with leading spaces. In this case, each value contains
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 contains 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, the Field lines displayed by myisamchk -dvv include additional
information about each column:
• Type
The data type. The value may contain any of the following descriptors:
• constant
All rows have the same value.
• 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 was 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 are stored using one bit.
• Huff tree
The number of the Huffman tree associated with the column.
• Bits
The number of bits used in the Huffman tree.
After you run myisampack, use myisamchk to re-create any indexes. At this time, you can also sort the
index blocks and create statistics needed for the MySQL optimizer to work more efficiently:
myisamchk -rq --sort-index --analyze tbl_name.MYI
After you have installed the packed table into the MySQL database directory, you should execute
mysqladmin flush-tables to force mysqld to start using the new table.
To unpack a packed table, use the --unpack option to myisamchk.
COPYRIGHT
Copyright © 1997, 2023, Oracle and/or its affiliates.
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.
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.
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 02110-1301 USA or see
http://www.gnu.org/licenses/.
SEE ALSO
For more information, please refer to the MySQL Reference Manual, which may already be installed locally
and which is also available online at http://dev.mysql.com/doc/.
AUTHOR
Oracle Corporation (http://dev.mysql.com/).