MySQL Manuel de Référence pour 3.23.5-alpha. - 12.5 Le générateur de tables compressée



	12.5 Le générateur de tables compressée `pack_isam` is an extra utility that you get when you order more than 10 licenses or extended support. Since `pack_isam` is distributed only in binary form, `pack_isam` is available only on some platforms. Of course, all future updates to `pack_isam` are included in the price. `pack_isam` may at some time be included as standard when we get some kind of turnover for MySQL. `pack_isam` works by compressing each column in the table separately. The information needed to decompress columns is read into memory when the table is opened. This results in much better performance when accessing individual records, since you only have to uncompress exactly one record, not a much larger disk block like when using Stacker on MS-DOS. Usually, `pack_isam` packs the data file 40%-70%. MySQL uses memory mapping (`mmap()`) on compressed tables and falls back to normal read/write file usage if `mmap()` doesn't work. There are currently two limitations with `pack_isam`: After packing, the table is read only. It can't pack `BLOB` columns, yet. Fixing these limitations is on our TODO list but with low priority. `pack_isam` is invoked like this: shell> pack_isam [options] filename ... Each filename should be the name of an index (`.ISM') file. If you are not in the database directory, you should specify the pathname to the file. It is permissible to omit the `.ISM' extension. `pack_isam` supports the following options: `-b, --backup` Make a backup of the table as `nom_table.OLD`. `-#, --debug=debug_options` Output debug log. The `debug_options` string often is `'d:t:o,filename'`. `-f, --force` Force packing of the table even if it becomes bigger or if the temporary file exists. (`pack_isam` creates a temporary file named `nom_table.TMD' while it compresses the table. If you kill `pack_isam`, the `.TMD' file may not be deleted. Normally, `pack_isam` exits with an error if it finds that `nom_table.TMD' exists. With `--force`, `pack_isam` packs the table anyway. `-?, --help` Display a help message and exit. `-j big_nom_table, --join=big_nom_table` Join all tables named on the command line into a single table `big_nom_table`. All tables that are to be combined MUST be identical (same column names and types, same indexes, etc.) `-p #, --packlength=#` Specify the record length storage size, in bytes. The value should be 1, 2 or 3. (`pack_isam` stores all rows with length pointers of 1, 2 or 3 bytes. In most normal cases, `pack_isam` 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, `pack_isam` will print a note that the next time you pack the same file, you could use a shorter record length.) `-s, --silent` Silent mode. Write output only when errors occur. `-t, --test` Don't pack table, only test packing it. `-T dir_name, --tmp_dir=dir_name` Use the named directory as the location in which to write the temporary table. `-v, --verbose` Verbose mode. Write info about progress and packing result. `-V, --version` Display version information and exit. `-w, --wait` Wait and retry if table is in use. If the `mysqld` server was invoked with the `--skip-locking` option, it is not a good idea to invoke `pack_isam` if the table might be updated during the packing process. The sequence of commands shown below illustrates a typical table compression session: shell> ls -l station.* -rw-rw-r-- 1 monty my 994128 Apr 17 19:00 station.ISD -rw-rw-r-- 1 monty my 53248 Apr 17 19:00 station.ISM -rw-rw-r-- 1 monty my 5767 Apr 17 19:00 station.frm shell> isamchk -dvv station ISAM 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> pack_isam station.ISM Compressing station.ISM: (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% shell> ls -l station.* -rw-rw-r-- 1 monty my 127874 Apr 17 19:00 station.ISD -rw-rw-r-- 1 monty my 55296 Apr 17 19:04 station.ISM -rw-rw-r-- 1 monty my 5767 Apr 17 19:00 station.frm shell> isamchk -dvv station ISAM 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 Datafilepointer (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 The information printed by `pack_isam` is described below: `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 1 bit. `empty-zero` The number of columns containing values that are only binary 0's; these will occupy 1 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, an `INTEGER` column may be changed to `MEDIUMINT`). `pre-space` The number of decimal columns that are stored with leading space. 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 space. 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, and that 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, `isamchk -dvv` prints additional information about each field: `Type` The field type may contain 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 end space 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` 0 values are stored in 1 bit. `Huff tree` The Huffman tree associated with the field `Bits` The number of bits used in the Huffman tree.