8.4. myisampack — Generate Compressed, Read-Only MyISAM Tables
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 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). 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 did not have this
capability.)
Invoke myisampack like this:
shell> myisampack [options] file_name ...
Each filename argument 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.
After you compress a table with myisampack,
you should use myisamchk -rq to rebuild its
indexes. Section 8.2, “myisamchk — MyISAM Table-Maintenance Utility”.
myisampack supports the following options:
-
--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=path
The directory where character sets are installed. See
Section 5.10.1, “The Character Set Used for Data and Sorting”.
-
--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 intermediate file from an earlier
invocation of myisampack exists.
(myisampack creates an intermediate
file named
tbl_name.TMD.TMD file might not be deleted.)
Normally, myisampack exits with an
error if it finds that
tbl_name.TMD--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=len,
-p len
Specify the row 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 correct 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 prints a note that you could
use a shorter row length the next time you pack the same
file.
-
--silent, -s
Silent mode. Write output only when errors occur.
-
--test, -t
Do not actually pack the table, just test packing it.
-
--tmpdir=path,
-T path
Use the named directory as the location where
myisamchk 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:
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 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, myisamchk
-dvv prints 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, 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
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.
