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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:
shell> 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. Section 4.6.4, “myisamchk — MyISAM Table-Maintenance Utility”.
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,-bMake a backup of each table's data file using the name
tbl_name.OLDThe directory where character sets are installed. See Section 10.14, “Character Set Configuration”.
--debug[=,debug_options]-# [debug_options]Write a debugging log. A typical
debug_optionsstring isd:t:o,. The default isfile_named:t:o.--force,-fProduce 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.TMDfile might not be deleted.) Normally, myisampack exits with an error if it finds thattbl_name.TMD--force, myisampack packs the table anyway.--join=,big_tbl_name-jbig_tbl_nameJoin 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_namemust not exist prior to the join operation. All source tables named on the command line to be merged intobig_tbl_namemust exist. The source tables are read for the join operation but not modified.--silent,-sSilent mode. Write output only when errors occur.
--test,-tDo not actually pack the table, just test packing it.
--tmpdir=,dir_name-Tdir_nameUse the named directory as the location where myisampack creates temporary files.
--verbose,-vVerbose mode. Write information about the progress of the packing operation and its result.
--version,-VDisplay version information and exit.
--wait,-wWait 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 jones my 994128 Apr 17 19:00 station.MYD
-rw-rw-r-- 1 jones my 53248 Apr 17 19:00 station.MYI
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> myisamchk -rq station
- check record delete-chain
- recovering (with sort) MyISAM-table 'station'
Data records: 1192
- Fixing index 1
- Fixing index 2
shell> mysqladmin -uroot flush-tables
shell> 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
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 9myisampack displays the following kinds of information:
normalThe number of columns for which no extra packing is used.
empty-spaceThe number of columns containing values that are only spaces. These occupy one bit.
empty-zeroThe number of columns containing values that are only binary zeros. These occupy one bit.
empty-fillThe 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
BIGINTcolumn (eight bytes) can be stored as aTINYINTcolumn (one byte) if all its values are in the range from-128to127.pre-spaceThe 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-spaceThe 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-lookupThe column had only a small number of different values, which were converted to an
ENUMbefore Huffman compression.zeroThe number of columns for which all values are zero.
Original treesThe initial number of Huffman trees.
After joinThe 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:
TypeThe data type. The value may contain any of the following descriptors:
constantAll rows have the same value.
no endspaceDo not store endspace.
no endspace, not_alwaysDo not store endspace and do not do endspace compression for all values.
no endspace, no emptyDo not store endspace. Do not store empty values.
table-lookupThe column was converted to an
ENUM.zerofill(N)The most significant
Nbytes in the value are always 0 and are not stored.no zerosDo not store zeros.
always zeroZero values are stored using one bit.
Huff treeThe number of the Huffman tree associated with the column.
BitsThe 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:
shell> myisamchk -rq --sort-index --analyze tbl_name.MYIAfter 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.
Document created the 26/06/2006, last modified the 26/10/2018
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