### 4.6.6 myisampack — Generate Compressed, Read-Only MyISAM Tables

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 The [**myisampack**](myisampack.html) utility compresses `MyISAM` tables. [**myisampack**](myisampack.html) works by compressing each column in the table separately. Usually, [**myisampack**](myisampack.html) 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**](mysqld.html) server was invoked with external locking disabled, it is not a good idea to invoke [**myisampack**](myisampack.html) 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**](myisampack.html) does not support partitioned tables.

 Invoke [**myisampack**](myisampack.html) 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**](myisampack.html), use [**myisamchk -rq**](myisamchk.html) to rebuild its indexes. [Section 4.6.4, “myisamchk — MyISAM Table-Maintenance Utility”](myisamchk.html).

[**myisampack**](myisampack.html) 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”](option-file-options.html).

* []() [`--help`](myisampack.html#option_myisampack_help), `-?`

  []()[]()

   Display a help message and exit.

* []() [`--backup`](myisampack.html#option_myisampack_backup), `-b`

  []()[]()

   Make a backup of each table's data file using the name `*`tbl_name`*.OLD`.

* []() [`--character-sets-dir=*`dir_name`*`](myisampack.html#option_myisampack_character-sets-dir)

  []()[]()

   The directory where character sets are installed. See [Section 10.15, “Character Set Configuration”](charset-configuration.html).

* []() [`--debug[=*`debug_options`*]`](myisampack.html#option_myisampack_debug), `-# [*`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`](source-configuration-options.html#option_cmake_with_debug). MySQL release binaries provided by Oracle are *not* built using this option.

* []() [`--force`](myisampack.html#option_myisampack_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**](myisampack.html) exists. ([**myisampack**](myisampack.html) creates an intermediate file named `*`tbl_name`*.TMD` in the database directory while it compresses the table. If you kill [**myisampack**](myisampack.html), the `.TMD` file might not be deleted.) Normally, [**myisampack**](myisampack.html) exits with an error if it finds that `*`tbl_name`*.TMD` exists. With [`--force`](myisampack.html#option_myisampack_force), [**myisampack**](myisampack.html) packs the table anyway.

* []() [`--join=*`big_tbl_name`*`](myisampack.html#option_myisampack_join), `-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`](myisampack.html#option_myisampack_silent), `-s`

  []()[]()

   Silent mode. Write output only when errors occur.

* []() [`--test`](myisampack.html#option_myisampack_test), `-t`

  []()[]()

   Do not actually pack the table, just test packing it.

* []() [`--tmpdir=*`dir_name`*`](myisampack.html#option_myisampack_tmpdir), `-T *`dir_name`*`

  []()[]()

   Use the named directory as the location where [**myisampack**](myisampack.html) creates temporary files.

* []() [`--verbose`](myisampack.html#option_myisampack_verbose), `-v`

  []()[]()

   Verbose mode. Write information about the progress of the packing operation and its result.

* []() [`--version`](myisampack.html#option_myisampack_version), `-V`

  []()[]()

   Display version information and exit.

* []() [`--wait`](myisampack.html#option_myisampack_wait), `-w`

  []()[]()

   Wait and retry if the table is in use. If the [**mysqld**](mysqld.html) server was invoked with external locking disabled, it is not a good idea to invoke [**myisampack**](myisampack.html) if the table might be updated by the server during the packing process.

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 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

$> 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

$> 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**](myisampack.html) 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`](integer-types.html) column (eight bytes) can be stored as a [`TINYINT`](integer-types.html) 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`](enum.html) 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**](myisamchk.html) 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`](enum.html).

  * `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**](myisampack.html), use [**myisamchk**](myisamchk.html) 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**](mysqladmin.html) to force [**mysqld**](mysqld.html) to start using the new table.

 To unpack a packed table, use the [`--unpack`](myisamchk-repair-options.html#option_myisamchk_unpack) option to [**myisamchk**](myisamchk.html).

