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8.2 `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'.
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