7.2 myisampack, the MySQL Compressed Read-Only Table Generator

7.2 myisampack , the MySQL Compressed Read-Only Table Generator

The myisampack utility compresses MyISAM tables. myisampack works by compressing each column in the table separately. Usually, myisampack packs the data file 40%-70%.

When the table is used later, the information needed to decompress columns is read into memory. 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.

MySQL uses mmap() when possible to perform memory mapping on compressed tables. If mmap() doesn't work, MySQL falls back to normal read/write file operations.

A similar utility, pack_isam , compresses ISAM tables. Because ISAM tables are deprecated, this section discusses only myisampack , but the general procedures for using myisampack are also true for pack_isam unless otherwise specified.

Please note the following:

• If the mysqld server was invoked with the --skip-external-locking option, it is not a good idea to invoke myisampack if the table might be updated by the server during the packing process.

• After packing a table, it becomes read-only. This is generally intended (such as when accessing packed tables on a CD). Allowing writes to a packed table is on our TODO list, but with low priority.

• myisampack can pack BLOB or TEXT columns. The older pack_isam program for ISAM tables cannot.

Invoke myisampack 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:

• --help , -?

  Display a help message and exit.

• --backup , -b

  Make a backup of the table data file using the name tbl_name .OLD .

• --debug[= debug_options ] , -# [ debug_options ]

  Write a debugging log. The debug_options string often is 'd:t:o, file_name ' .

• --force , -f

  Produce a packed table even if it becomes larger than the original or if the temporary file from an earlier invocation of myisampack exists. ( myisampack creates a temporary file named tbl_name .TMD 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 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).

• --packlength= # , -p #

  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.

• --silent , -s

  Silent mode. Write output only when errors occur.

• --test , -t

  Don't actually pack the table, just test packing it.

• --tmp_dir= path , -T path

  Use the named directory as the location in which to write the temporary table.

• --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 the --skip-external-locking option, 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:

 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% 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 

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 will occupy one bit.

• empty-zero

  The number of columns containing values that are only binary zeros; these will occupy one 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, 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 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 column:

• Type

  The column type. The value may contain any of 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 endspace 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

    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 , you must run 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 :

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

A similar procedure applies for ISAM tables. After using pack_isam , use isamchk to re-create the indexes:

 shell>  isamchk -rq --sort-index --analyze   tbl_name   .ISM  

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 or isamchk .