1MYISAMCHK(1)                 MySQL Database System                MYISAMCHK(1)
2
3
4

NAME

6       myisamchk - MyISAM table-maintenance utility
7

SYNOPSIS

9       myisamchk [options] tbl_name ...
10

DESCRIPTION

12       The myisamchk utility gets information about your database tables or
13       checks, repairs, or optimizes them.  myisamchk works with MyISAM tables
14       (tables that have .MYD and .MYI files for storing data and indexes).
15
16       You can also use the CHECK TABLE and REPAIR TABLE statements to check
17       and repair MyISAM tables. See Section 13.7.3.2, “CHECK TABLE
18       Statement”, and Section 13.7.3.5, “REPAIR TABLE Statement”.
19
20       The use of myisamchk with partitioned tables is not supported.
21
22           Caution
23           It is best to make a backup of a table before performing a table
24           repair operation; under some circumstances the operation might
25           cause data loss. Possible causes include but are not limited to
26           file system errors.
27
28       Invoke myisamchk like this:
29
30           myisamchk [options] tbl_name ...
31
32       The options specify what you want myisamchk to do. They are described
33       in the following sections. You can also get a list of options by
34       invoking myisamchk --help.
35
36       With no options, myisamchk simply checks your table as the default
37       operation. To get more information or to tell myisamchk to take
38       corrective action, specify options as described in the following
39       discussion.
40
41       tbl_name is the database table you want to check or repair. If you run
42       myisamchk somewhere other than in the database directory, you must
43       specify the path to the database directory, because myisamchk has no
44       idea where the database is located. In fact, myisamchk does not
45       actually care whether the files you are working on are located in a
46       database directory. You can copy the files that correspond to a
47       database table into some other location and perform recovery operations
48       on them there.
49
50       You can name several tables on the myisamchk command line if you wish.
51       You can also specify a table by naming its index file (the file with
52       the .MYI suffix). This enables you to specify all tables in a directory
53       by using the pattern *.MYI. For example, if you are in a database
54       directory, you can check all the MyISAM tables in that directory like
55       this:
56
57           myisamchk *.MYI
58
59       If you are not in the database directory, you can check all the tables
60       there by specifying the path to the directory:
61
62           myisamchk /path/to/database_dir/*.MYI
63
64       You can even check all tables in all databases by specifying a wildcard
65       with the path to the MySQL data directory:
66
67           myisamchk /path/to/datadir/*/*.MYI
68
69       The recommended way to quickly check all MyISAM tables is:
70
71           myisamchk --silent --fast /path/to/datadir/*/*.MYI
72
73       If you want to check all MyISAM tables and repair any that are
74       corrupted, you can use the following command:
75
76           myisamchk --silent --force --fast --update-state \
77                     --key_buffer_size=64M --myisam_sort_buffer_size=64M \
78                     --read_buffer_size=1M --write_buffer_size=1M \
79                     /path/to/datadir/*/*.MYI
80
81       This command assumes that you have more than 64MB free. For more
82       information about memory allocation with myisamchk, see the section
83       called “MYISAMCHK MEMORY USAGE”.
84
85       For additional information about using myisamchk, see Section 7.6,
86       “MyISAM Table Maintenance and Crash Recovery”.
87
88           Important
89           You must ensure that no other program is using the tables while you
90           are running myisamchk. The most effective means of doing so is to
91           shut down the MySQL server while running myisamchk, or to lock all
92           tables that myisamchk is being used on.
93
94           Otherwise, when you run myisamchk, it may display the following
95           error message:
96
97               warning: clients are using or haven't closed the table properly
98
99           This means that you are trying to check a table that has been
100           updated by another program (such as the mysqld server) that hasn't
101           yet closed the file or that has died without closing the file
102           properly, which can sometimes lead to the corruption of one or more
103           MyISAM tables.
104
105           If mysqld is running, you must force it to flush any table
106           modifications that are still buffered in memory by using FLUSH
107           TABLES. You should then ensure that no one is using the tables
108           while you are running myisamchk
109
110           However, the easiest way to avoid this problem is to use CHECK
111           TABLE instead of myisamchk to check tables. See Section 13.7.3.2,
112           “CHECK TABLE Statement”.
113
114       myisamchk supports the following options, which can be specified on the
115       command line or in the [myisamchk] group of an option file. For
116       information about option files used by MySQL programs, see
117       Section 4.2.2.2, “Using Option Files”.
118

MYISAMCHK GENERAL OPTIONS

120       The options described in this section can be used for any type of table
121       maintenance operation performed by myisamchk. The sections following
122       this one describe options that pertain only to specific operations,
123       such as table checking or repairing.
124
125--help, -?  Display a help message and exit. Options are grouped by
126           type of operation.
127
128--HELP, -H Display a help message and exit. Options are presented
129           in a single list.
130
131--debug=debug_options, -# debug_options Write a debugging log. A
132           typical debug_options string is d:t:o,file_name. The default is
133           d:t:o,/tmp/myisamchk.trace.
134
135           This option is available only if MySQL was built using WITH_DEBUG.
136           MySQL release binaries provided by Oracle are not built using this
137           option.
138
139--defaults-extra-file=file_name Read this option file after the
140           global option file but (on Unix) before the user option file. If
141           the file does not exist or is otherwise inaccessible, an error
142           occurs. If file_name is not an absolute path name, it is
143           interpreted relative to the current directory.
144
145           For additional information about this and other option-file
146           options, see Section 4.2.2.3, “Command-Line Options that Affect
147           Option-File Handling”.
148
149--defaults-file=file_name Use only the given option file. If the
150           file does not exist or is otherwise inaccessible, an error occurs.
151           If file_name is not an absolute path name, it is interpreted
152           relative to the current directory.
153
154           For additional information about this and other option-file
155           options, see Section 4.2.2.3, “Command-Line Options that Affect
156           Option-File Handling”.
157
158--defaults-group-suffix=str Read not only the usual option groups,
159           but also groups with the usual names and a suffix of str. For
160           example, myisamchk normally reads the [myisamchk] group. If this
161           option is given as --defaults-group-suffix=_other, myisamchk also
162           reads the [myisamchk_other] group.
163
164           For additional information about this and other option-file
165           options, see Section 4.2.2.3, “Command-Line Options that Affect
166           Option-File Handling”.
167
168--no-defaults Do not read any option files. If program startup
169           fails due to reading unknown options from an option file,
170           --no-defaults can be used to prevent them from being read.
171
172           The exception is that the .mylogin.cnf file is read in all cases,
173           if it exists. This permits passwords to be specified in a safer way
174           than on the command line even when --no-defaults is used. To create
175           .mylogin.cnf, use the mysql_config_editor utility. See
176           mysql_config_editor(1).
177
178           For additional information about this and other option-file
179           options, see Section 4.2.2.3, “Command-Line Options that Affect
180           Option-File Handling”.
181
182--print-defaults Print the program name and all options that it
183           gets from option files.
184
185           For additional information about this and other option-file
186           options, see Section 4.2.2.3, “Command-Line Options that Affect
187           Option-File Handling”.
188
189--silent, -s Silent mode. Write output only when errors occur. You
190           can use -s twice (-ss) to make myisamchk very silent.
191
192--verbose, -v Verbose mode. Print more information about what the
193           program does. This can be used with -d and -e. Use -v multiple
194           times (-vv, -vvv) for even more output.
195
196--version, -V Display version information and exit.
197
198--wait, -w Instead of terminating with an error if the table is
199           locked, wait until the table is unlocked before continuing. If you
200           are running mysqld with external locking disabled, the table can be
201           locked only by another myisamchk command.
202
203       You can also set the following variables by using --var_name=value
204       syntax:
205
206       ┌───────────────────────┬───────────────────┐
207Variable               Default Value     
208       ├───────────────────────┼───────────────────┤
209       │decode_bits            │ 9                 │
210       ├───────────────────────┼───────────────────┤
211       │ft_max_word_len        │ version-dependent │
212       ├───────────────────────┼───────────────────┤
213       │ft_min_word_len        │ 4                 │
214       ├───────────────────────┼───────────────────┤
215       │ft_stopword_file       │ built-in list     │
216       ├───────────────────────┼───────────────────┤
217       │key_buffer_size        │ 523264            │
218       ├───────────────────────┼───────────────────┤
219       │myisam_block_size      │ 1024              │
220       ├───────────────────────┼───────────────────┤
221       │myisam_sort_key_blocks │ 16                │
222       ├───────────────────────┼───────────────────┤
223       │read_buffer_size       │ 262136            │
224       ├───────────────────────┼───────────────────┤
225       │sort_buffer_size       │ 2097144           │
226       ├───────────────────────┼───────────────────┤
227       │sort_key_blocks        │ 16                │
228       ├───────────────────────┼───────────────────┤
229       │stats_method           │ nulls_unequal     │
230       ├───────────────────────┼───────────────────┤
231       │write_buffer_size      │ 262136            │
232       └───────────────────────┴───────────────────┘
233
234       The possible myisamchk variables and their default values can be
235       examined with myisamchk --help:
236
237       myisam_sort_buffer_size is used when the keys are repaired by sorting
238       keys, which is the normal case when you use --recover.
239       sort_buffer_size is a deprecated synonym for myisam_sort_buffer_size.
240
241       key_buffer_size is used when you are checking the table with
242       --extend-check or when the keys are repaired by inserting keys row by
243       row into the table (like when doing normal inserts). Repairing through
244       the key buffer is used in the following cases:
245
246       •   You use --safe-recover.
247
248       •   The temporary files needed to sort the keys would be more than
249           twice as big as when creating the key file directly. This is often
250           the case when you have large key values for CHAR, VARCHAR, or TEXT
251           columns, because the sort operation needs to store the complete key
252           values as it proceeds. If you have lots of temporary space and you
253           can force myisamchk to repair by sorting, you can use the
254           --sort-recover option.
255
256       Repairing through the key buffer takes much less disk space than using
257       sorting, but is also much slower.
258
259       If you want a faster repair, set the key_buffer_size and
260       myisam_sort_buffer_size variables to about 25% of your available
261       memory. You can set both variables to large values, because only one of
262       them is used at a time.
263
264       myisam_block_size is the size used for index blocks.
265
266       stats_method influences how NULL values are treated for index
267       statistics collection when the --analyze option is given. It acts like
268       the myisam_stats_method system variable. For more information, see the
269       description of myisam_stats_method in Section 5.1.8, “Server System
270       Variables”, and Section 8.3.8, “InnoDB and MyISAM Index Statistics
271       Collection”.
272
273       ft_min_word_len and ft_max_word_len indicate the minimum and maximum
274       word length for FULLTEXT indexes on MyISAM tables.  ft_stopword_file
275       names the stopword file. These need to be set under the following
276       circumstances.
277
278       If you use myisamchk to perform an operation that modifies table
279       indexes (such as repair or analyze), the FULLTEXT indexes are rebuilt
280       using the default full-text parameter values for minimum and maximum
281       word length and the stopword file unless you specify otherwise. This
282       can result in queries failing.
283
284       The problem occurs because these parameters are known only by the
285       server. They are not stored in MyISAM index files. To avoid the problem
286       if you have modified the minimum or maximum word length or the stopword
287       file in the server, specify the same ft_min_word_len, ft_max_word_len,
288       and ft_stopword_file values to myisamchk that you use for mysqld. For
289       example, if you have set the minimum word length to 3, you can repair a
290       table with myisamchk like this:
291
292           myisamchk --recover --ft_min_word_len=3 tbl_name.MYI
293
294       To ensure that myisamchk and the server use the same values for
295       full-text parameters, you can place each one in both the [mysqld] and
296       [myisamchk] sections of an option file:
297
298           [mysqld]
299           ft_min_word_len=3
300           [myisamchk]
301           ft_min_word_len=3
302
303       An alternative to using myisamchk is to use the REPAIR TABLE, ANALYZE
304       TABLE, OPTIMIZE TABLE, or ALTER TABLE. These statements are performed
305       by the server, which knows the proper full-text parameter values to
306       use.
307

MYISAMCHK CHECK OPTIONS

309       myisamchk supports the following options for table checking operations:
310
311--check, -c Check the table for errors. This is the default
312           operation if you specify no option that selects an operation type
313           explicitly.
314
315--check-only-changed, -C Check only tables that have changed since
316           the last check.
317
318--extend-check, -e Check the table very thoroughly. This is quite
319           slow if the table has many indexes. This option should only be used
320           in extreme cases. Normally, myisamchk or myisamchk --medium-check
321           should be able to determine whether there are any errors in the
322           table.
323
324           If you are using --extend-check and have plenty of memory, setting
325           the key_buffer_size variable to a large value helps the repair
326           operation run faster.
327
328           See also the description of this option under table repair options.
329
330           For a description of the output format, see the section called
331           “OBTAINING TABLE INFORMATION WITH MYISAMCHK”.
332
333--fast, -F Check only tables that haven't been closed properly.
334
335--force, -f Do a repair operation automatically if myisamchk finds
336           any errors in the table. The repair type is the same as that
337           specified with the --recover or -r option.
338
339--information, -i Print informational statistics about the table
340           that is checked.
341
342--medium-check, -m Do a check that is faster than an --extend-check
343           operation. This finds only 99.99% of all errors, which should be
344           good enough in most cases.
345
346--read-only, -T Do not mark the table as checked. This is useful if
347           you use myisamchk to check a table that is in use by some other
348           application that does not use locking, such as mysqld when run with
349           external locking disabled.
350
351--update-state, -U Store information in the .MYI file to indicate
352           when the table was checked and whether the table crashed. This
353           should be used to get full benefit of the --check-only-changed
354           option, but you shouldn't use this option if the mysqld server is
355           using the table and you are running it with external locking
356           disabled.
357

MYISAMCHK REPAIR OPTIONS

359       myisamchk supports the following options for table repair operations
360       (operations performed when an option such as --recover or
361       --safe-recover is given):
362
363--backup, -B Make a backup of the .MYD file as file_name-time.BAK
364
365--character-sets-dir=dir_name The directory where character sets
366           are installed. See Section 10.15, “Character Set Configuration”.
367
368--correct-checksum Correct the checksum information for the table.
369
370--data-file-length=len, -D len The maximum length of the data file
371           (when re-creating data file when it is “full”).
372
373--extend-check, -e
374
375           Do a repair that tries to recover every possible row from the data
376           file. Normally, this also finds a lot of garbage rows. Do not use
377           this option unless you are desperate.
378
379           See also the description of this option under table checking
380           options.
381
382           For a description of the output format, see the section called
383           “OBTAINING TABLE INFORMATION WITH MYISAMCHK”.
384
385--force, -f
386
387           Overwrite old intermediate files (files with names like
388           tbl_name.TMD) instead of aborting.
389
390--keys-used=val, -k val For myisamchk, the option value is a bit
391           value that indicates which indexes to update. Each binary bit of
392           the option value corresponds to a table index, where the first
393           index is bit 0. An option value of 0 disables updates to all
394           indexes, which can be used to get faster inserts. Deactivated
395           indexes can be reactivated by using myisamchk -r.
396
397--no-symlinks, -l Do not follow symbolic links. Normally myisamchk
398           repairs the table that a symlink points to. This option does not
399           exist as of MySQL 4.0 because versions from 4.0 on do not remove
400           symlinks during repair operations.
401
402--max-record-length=len Skip rows larger than the given length if
403           myisamchk cannot allocate memory to hold them.
404
405--parallel-recover, -p
406
407               Note
408               This option is deprecated in MySQL 8.0.28 and removed in MySQL
409               8.0.30.
410           Use the same technique as -r and -n, but create all the keys in
411           parallel, using different threads.  This is beta-quality code. Use
412           at your own risk!
413
414--quick, -q Achieve a faster repair by modifying only the index
415           file, not the data file. You can specify this option twice to force
416           myisamchk to modify the original data file in case of duplicate
417           keys.
418
419--recover, -r Do a repair that can fix almost any problem except
420           unique keys that are not unique (which is an extremely unlikely
421           error with MyISAM tables). If you want to recover a table, this is
422           the option to try first. You should try --safe-recover only if
423           myisamchk reports that the table cannot be recovered using
424           --recover. (In the unlikely case that --recover fails, the data
425           file remains intact.)
426
427           If you have lots of memory, you should increase the value of
428           myisam_sort_buffer_size.
429
430--safe-recover, -o Do a repair using an old recovery method that
431           reads through all rows in order and updates all index trees based
432           on the rows found. This is an order of magnitude slower than
433           --recover, but can handle a couple of very unlikely cases that
434           --recover cannot. This recovery method also uses much less disk
435           space than --recover. Normally, you should repair first using
436           --recover, and then with --safe-recover only if --recover fails.
437
438           If you have lots of memory, you should increase the value of
439           key_buffer_size.
440
441--set-collation=name Specify the collation to use for sorting table
442           indexes. The character set name is implied by the first part of the
443           collation name.
444
445--sort-recover, -n Force myisamchk to use sorting to resolve the
446           keys even if the temporary files would be very large.
447
448--tmpdir=dir_name, -t dir_name The path of the directory to be used
449           for storing temporary files. If this is not set, myisamchk uses the
450           value of the TMPDIR environment variable.  --tmpdir can be set to a
451           list of directory paths that are used successively in round-robin
452           fashion for creating temporary files. The separator character
453           between directory names is the colon (:) on Unix and the semicolon
454           (;) on Windows.
455
456--unpack, -u Unpack a table that was packed with myisampack.
457

OTHER MYISAMCHK OPTIONS

459       myisamchk supports the following options for actions other than table
460       checks and repairs:
461
462--analyze, -a Analyze the distribution of key values. This improves
463           join performance by enabling the join optimizer to better choose
464           the order in which to join the tables and which indexes it should
465           use. To obtain information about the key distribution, use a
466           myisamchk --description --verbose tbl_name command or the SHOW
467           INDEX FROM tbl_name statement.
468
469--block-search=offset, -b offset Find the record that a block at
470           the given offset belongs to.
471
472--description, -d Print some descriptive information about the
473           table. Specifying the --verbose option once or twice produces
474           additional information. See the section called “OBTAINING TABLE
475           INFORMATION WITH MYISAMCHK”.
476
477--set-auto-increment[=value], -A[value] Force AUTO_INCREMENT
478           numbering for new records to start at the given value (or higher,
479           if there are existing records with AUTO_INCREMENT values this
480           large). If value is not specified, AUTO_INCREMENT numbers for new
481           records begin with the largest value currently in the table, plus
482           one.
483
484--sort-index, -S Sort the index tree blocks in high-low order. This
485           optimizes seeks and makes table scans that use indexes faster.
486
487--sort-records=N, -R N Sort records according to a particular
488           index. This makes your data much more localized and may speed up
489           range-based SELECT and ORDER BY operations that use this index.
490           (The first time you use this option to sort a table, it may be very
491           slow.) To determine a table's index numbers, use SHOW INDEX, which
492           displays a table's indexes in the same order that myisamchk sees
493           them. Indexes are numbered beginning with 1.
494
495           If keys are not packed (PACK_KEYS=0), they have the same length, so
496           when myisamchk sorts and moves records, it just overwrites record
497           offsets in the index. If keys are packed (PACK_KEYS=1), myisamchk
498           must unpack key blocks first, then re-create indexes and pack the
499           key blocks again. (In this case, re-creating indexes is faster than
500           updating offsets for each index.)
501

OBTAINING TABLE INFORMATION WITH MYISAMCHK

503       To obtain a description of a MyISAM table or statistics about it, use
504       the commands shown here. The output from these commands is explained
505       later in this section.
506
507myisamchk -d tbl_name
508
509           Runs myisamchk in “describe mode” to produce a description of your
510           table. If you start the MySQL server with external locking
511           disabled, myisamchk may report an error for a table that is updated
512           while it runs. However, because myisamchk does not change the table
513           in describe mode, there is no risk of destroying data.
514
515myisamchk -dv tbl_name
516
517           Adding -v runs myisamchk in verbose mode so that it produces more
518           information about the table. Adding -v a second time produces even
519           more information.
520
521myisamchk -eis tbl_name
522
523           Shows only the most important information from a table. This
524           operation is slow because it must read the entire table.
525
526myisamchk -eiv tbl_name
527
528           This is like -eis, but tells you what is being done.
529
530       The tbl_name argument can be either the name of a MyISAM table or the
531       name of its index file, as described in myisamchk(1). Multiple tbl_name
532       arguments can be given.
533
534       Suppose that a table named person has the following structure. (The
535       MAX_ROWS table option is included so that in the example output from
536       myisamchk shown later, some values are smaller and fit the output
537       format more easily.)
538
539           CREATE TABLE person
540           (
541             id         INT NOT NULL AUTO_INCREMENT,
542             last_name  VARCHAR(20) NOT NULL,
543             first_name VARCHAR(20) NOT NULL,
544             birth      DATE,
545             death      DATE,
546             PRIMARY KEY (id),
547             INDEX (last_name, first_name),
548             INDEX (birth)
549           ) MAX_ROWS = 1000000 ENGINE=MYISAM;
550
551       Suppose also that the table has these data and index file sizes:
552
553           -rw-rw----  1 mysql  mysql  9347072 Aug 19 11:47 person.MYD
554           -rw-rw----  1 mysql  mysql  6066176 Aug 19 11:47 person.MYI
555
556       Example of myisamchk -dvv output:
557
558           MyISAM file:         person
559           Record format:       Packed
560           Character set:       utf8mb4_0900_ai_ci (255)
561           File-version:        1
562           Creation time:       2017-03-30 21:21:30
563           Status:              checked,analyzed,optimized keys,sorted index pages
564           Auto increment key:              1  Last value:                306688
565           Data records:               306688  Deleted blocks:                 0
566           Datafile parts:             306688  Deleted data:                   0
567           Datafile pointer (bytes):        4  Keyfile pointer (bytes):        3
568           Datafile length:           9347072  Keyfile length:           6066176
569           Max datafile length:    4294967294  Max keyfile length:   17179868159
570           Recordlength:                   54
571           table description:
572           Key Start Len Index   Type                     Rec/key         Root  Blocksize
573           1   2     4   unique  long                           1                    1024
574           2   6     80  multip. varchar prefix                 0                    1024
575               87    80          varchar                        0
576           3   168   3   multip. uint24 NULL                    0                    1024
577           Field Start Length Nullpos Nullbit Type
578           1     1     1
579           2     2     4                      no zeros
580           3     6     81                     varchar
581           4     87    81                     varchar
582           5     168   3      1       1       no zeros
583           6     171   3      1       2       no zeros
584
585       Explanations for the types of information myisamchk produces are given
586       here.  “Keyfile” refers to the index file.  “Record” and “row” are
587       synonymous, as are “field” and “column.”
588
589       The initial part of the table description contains these values:
590
591       •   MyISAM file
592
593           Name of the MyISAM (index) file.
594
595       •   Record format
596
597           The format used to store table rows. The preceding examples use
598           Fixed length. Other possible values are Compressed and Packed.
599           (Packed corresponds to what SHOW TABLE STATUS reports as Dynamic.)
600
601       •   Chararacter set
602
603           The table default character set.
604
605       •   File-version
606
607           Version of MyISAM format. Always 1.
608
609       •   Creation time
610
611           When the data file was created.
612
613       •   Recover time
614
615           When the index/data file was last reconstructed.
616
617       •   Status
618
619           Table status flags. Possible values are crashed, open, changed,
620           analyzed, optimized keys, and sorted index pages.
621
622       •   Auto increment key, Last value
623
624           The key number associated the table's AUTO_INCREMENT column, and
625           the most recently generated value for this column. These fields do
626           not appear if there is no such column.
627
628       •   Data records
629
630           The number of rows in the table.
631
632       •   Deleted blocks
633
634           How many deleted blocks still have reserved space. You can optimize
635           your table to minimize this space. See Section 7.6.4, “MyISAM Table
636           Optimization”.
637
638       •   Datafile parts
639
640           For dynamic-row format, this indicates how many data blocks there
641           are. For an optimized table without fragmented rows, this is the
642           same as Data records.
643
644       •   Deleted data
645
646           How many bytes of unreclaimed deleted data there are. You can
647           optimize your table to minimize this space. See Section 7.6.4,
648           “MyISAM Table Optimization”.
649
650       •   Datafile pointer
651
652           The size of the data file pointer, in bytes. It is usually 2, 3, 4,
653           or 5 bytes. Most tables manage with 2 bytes, but this cannot be
654           controlled from MySQL yet. For fixed tables, this is a row address.
655           For dynamic tables, this is a byte address.
656
657       •   Keyfile pointer
658
659           The size of the index file pointer, in bytes. It is usually 1, 2,
660           or 3 bytes. Most tables manage with 2 bytes, but this is calculated
661           automatically by MySQL. It is always a block address.
662
663       •   Max datafile length
664
665           How long the table data file can become, in bytes.
666
667       •   Max keyfile length
668
669           How long the table index file can become, in bytes.
670
671       •   Recordlength
672
673           How much space each row takes, in bytes.
674
675       The table description part of the output includes a list of all keys in
676       the table. For each key, myisamchk displays some low-level information:
677
678       •   Key
679
680           This key's number. This value is shown only for the first column of
681           the key. If this value is missing, the line corresponds to the
682           second or later column of a multiple-column key. For the table
683           shown in the example, there are two table description lines for the
684           second index. This indicates that it is a multiple-part index with
685           two parts.
686
687       •   Start
688
689           Where in the row this portion of the index starts.
690
691       •   Len
692
693           How long this portion of the index is. For packed numbers, this
694           should always be the full length of the column. For strings, it may
695           be shorter than the full length of the indexed column, because you
696           can index a prefix of a string column. The total length of a
697           multiple-part key is the sum of the Len values for all key parts.
698
699       •   Index
700
701           Whether a key value can exist multiple times in the index. Possible
702           values are unique or multip.  (multiple).
703
704       •   Type
705
706           What data type this portion of the index has. This is a MyISAM data
707           type with the possible values packed, stripped, or empty.
708
709       •   Root
710
711           Address of the root index block.
712
713       •   Blocksize
714
715           The size of each index block. By default this is 1024, but the
716           value may be changed at compile time when MySQL is built from
717           source.
718
719       •   Rec/key
720
721           This is a statistical value used by the optimizer. It tells how
722           many rows there are per value for this index. A unique index always
723           has a value of 1. This may be updated after a table is loaded (or
724           greatly changed) with myisamchk -a. If this is not updated at all,
725           a default value of 30 is given.
726
727       The last part of the output provides information about each column:
728
729       •   Field
730
731           The column number.
732
733       •   Start
734
735           The byte position of the column within table rows.
736
737       •   Length
738
739           The length of the column in bytes.
740
741       •   Nullpos, Nullbit
742
743           For columns that can be NULL, MyISAM stores NULL values as a flag
744           in a byte. Depending on how many nullable columns there are, there
745           can be one or more bytes used for this purpose. The Nullpos and
746           Nullbit values, if nonempty, indicate which byte and bit contains
747           that flag indicating whether the column is NULL.
748
749           The position and number of bytes used to store NULL flags is shown
750           in the line for field 1. This is why there are six Field lines for
751           the person table even though it has only five columns.
752
753       •   Type
754
755           The data type. The value may contain any of the following
756           descriptors:
757
758           •   constant
759
760               All rows have the same value.
761
762           •   no endspace
763
764               Do not store endspace.
765
766           •   no endspace, not_always
767
768               Do not store endspace and do not do endspace compression for
769               all values.
770
771           •   no endspace, no empty
772
773               Do not store endspace. Do not store empty values.
774
775           •   table-lookup
776
777               The column was converted to an ENUM.
778
779           •   zerofill(N)
780
781               The most significant N bytes in the value are always 0 and are
782               not stored.
783
784           •   no zeros
785
786               Do not store zeros.
787
788           •   always zero
789
790               Zero values are stored using one bit.
791
792       •   Huff tree
793
794           The number of the Huffman tree associated with the column.
795
796       •   Bits
797
798           The number of bits used in the Huffman tree.
799
800       The Huff tree and Bits fields are displayed if the table has been
801       compressed with myisampack. See myisampack(1), for an example of this
802       information.
803
804       Example of myisamchk -eiv output:
805
806           Checking MyISAM file: person
807           Data records:  306688   Deleted blocks:       0
808           - check file-size
809           - check record delete-chain
810           No recordlinks
811           - check key delete-chain
812           block_size 1024:
813           - check index reference
814           - check data record references index: 1
815           Key:  1:  Keyblocks used:  98%  Packed:    0%  Max levels:  3
816           - check data record references index: 2
817           Key:  2:  Keyblocks used:  99%  Packed:   97%  Max levels:  3
818           - check data record references index: 3
819           Key:  3:  Keyblocks used:  98%  Packed:  -14%  Max levels:  3
820           Total:    Keyblocks used:  98%  Packed:   89%
821           - check records and index references
822           *** LOTS OF ROW NUMBERS DELETED ***
823           Records:            306688  M.recordlength:       25  Packed:            83%
824           Recordspace used:       97% Empty space:           2% Blocks/Record:   1.00
825           Record blocks:      306688  Delete blocks:         0
826           Record data:       7934464  Deleted data:          0
827           Lost space:         256512  Linkdata:        1156096
828           User time 43.08, System time 1.68
829           Maximum resident set size 0, Integral resident set size 0
830           Non-physical pagefaults 0, Physical pagefaults 0, Swaps 0
831           Blocks in 0 out 7, Messages in 0 out 0, Signals 0
832           Voluntary context switches 0, Involuntary context switches 0
833           Maximum memory usage: 1046926 bytes (1023k)
834
835       myisamchk -eiv output includes the following information:
836
837       •   Data records
838
839           The number of rows in the table.
840
841       •   Deleted blocks
842
843           How many deleted blocks still have reserved space. You can optimize
844           your table to minimize this space. See Section 7.6.4, “MyISAM Table
845           Optimization”.
846
847       •   Key
848
849           The key number.
850
851       •   Keyblocks used
852
853           What percentage of the keyblocks are used. When a table has just
854           been reorganized with myisamchk, the values are very high (very
855           near theoretical maximum).
856
857       •   Packed
858
859           MySQL tries to pack key values that have a common suffix. This can
860           only be used for indexes on CHAR and VARCHAR columns. For long
861           indexed strings that have similar leftmost parts, this can
862           significantly reduce the space used. In the preceding example, the
863           second key is 40 bytes long and a 97% reduction in space is
864           achieved.
865
866       •   Max levels
867
868           How deep the B-tree for this key is. Large tables with long key
869           values get high values.
870
871       •   Records
872
873           How many rows are in the table.
874
875       •   M.recordlength
876
877           The average row length. This is the exact row length for tables
878           with fixed-length rows, because all rows have the same length.
879
880       •   Packed
881
882           MySQL strips spaces from the end of strings. The Packed value
883           indicates the percentage of savings achieved by doing this.
884
885       •   Recordspace used
886
887           What percentage of the data file is used.
888
889       •   Empty space
890
891           What percentage of the data file is unused.
892
893       •   Blocks/Record
894
895           Average number of blocks per row (that is, how many links a
896           fragmented row is composed of). This is always 1.0 for fixed-format
897           tables. This value should stay as close to 1.0 as possible. If it
898           gets too large, you can reorganize the table. See Section 7.6.4,
899           “MyISAM Table Optimization”.
900
901       •   Recordblocks
902
903           How many blocks (links) are used. For fixed-format tables, this is
904           the same as the number of rows.
905
906       •   Deleteblocks
907
908           How many blocks (links) are deleted.
909
910       •   Recorddata
911
912           How many bytes in the data file are used.
913
914       •   Deleted data
915
916           How many bytes in the data file are deleted (unused).
917
918       •   Lost space
919
920           If a row is updated to a shorter length, some space is lost. This
921           is the sum of all such losses, in bytes.
922
923       •   Linkdata
924
925           When the dynamic table format is used, row fragments are linked
926           with pointers (4 to 7 bytes each).  Linkdata is the sum of the
927           amount of storage used by all such pointers.
928

MYISAMCHK MEMORY USAGE

930       Memory allocation is important when you run myisamchk.  myisamchk uses
931       no more memory than its memory-related variables are set to. If you are
932       going to use myisamchk on very large tables, you should first decide
933       how much memory you want it to use. The default is to use only about
934       3MB to perform repairs. By using larger values, you can get myisamchk
935       to operate faster. For example, if you have more than 512MB RAM
936       available, you could use options such as these (in addition to any
937       other options you might specify):
938
939           myisamchk --myisam_sort_buffer_size=256M \
940                      --key_buffer_size=512M \
941                      --read_buffer_size=64M \
942                      --write_buffer_size=64M ...
943
944       Using --myisam_sort_buffer_size=16M is probably enough for most cases.
945
946       Be aware that myisamchk uses temporary files in TMPDIR. If TMPDIR
947       points to a memory file system, out of memory errors can easily occur.
948       If this happens, run myisamchk with the --tmpdir=dir_name option to
949       specify a directory located on a file system that has more space.
950
951       When performing repair operations, myisamchk also needs a lot of disk
952       space:
953
954       •   Twice the size of the data file (the original file and a copy).
955           This space is not needed if you do a repair with --quick; in this
956           case, only the index file is re-created.  This space must be
957           available on the same file system as the original data file, as the
958           copy is created in the same directory as the original.
959
960       •   Space for the new index file that replaces the old one. The old
961           index file is truncated at the start of the repair operation, so
962           you usually ignore this space. This space must be available on the
963           same file system as the original data file.
964
965       •   When using --recover or --sort-recover (but not when using
966           --safe-recover), you need space on disk for sorting. This space is
967           allocated in the temporary directory (specified by TMPDIR or
968           --tmpdir=dir_name). The following formula yields the amount of
969           space required:
970
971               (largest_key + row_pointer_length) * number_of_rows * 2
972
973           You can check the length of the keys and the row_pointer_length
974           with myisamchk -dv tbl_name (see the section called “OBTAINING
975           TABLE INFORMATION WITH MYISAMCHK”). The row_pointer_length and
976           number_of_rows values are the Datafile pointer and Data records
977           values in the table description. To determine the largest_key
978           value, check the Key lines in the table description. The Len column
979           indicates the number of bytes for each key part. For a
980           multiple-column index, the key size is the sum of the Len values
981           for all key parts.
982
983       If you have a problem with disk space during repair, you can try
984       --safe-recover instead of --recover.
985
987       Copyright © 1997, 2022, Oracle and/or its affiliates.
988
989       This documentation is free software; you can redistribute it and/or
990       modify it only under the terms of the GNU General Public License as
991       published by the Free Software Foundation; version 2 of the License.
992
993       This documentation is distributed in the hope that it will be useful,
994       but WITHOUT ANY WARRANTY; without even the implied warranty of
995       MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE. See the GNU
996       General Public License for more details.
997
998       You should have received a copy of the GNU General Public License along
999       with the program; if not, write to the Free Software Foundation, Inc.,
1000       51 Franklin Street, Fifth Floor, Boston, MA 02110-1301 USA or see
1001       http://www.gnu.org/licenses/.
1002
1003

SEE ALSO

1005       For more information, please refer to the MySQL Reference Manual, which
1006       may already be installed locally and which is also available online at
1007       http://dev.mysql.com/doc/.
1008

AUTHOR

1010       Oracle Corporation (http://dev.mysql.com/).
1011
1012
1013
1014MySQL 8.0                         08/29/2022                      MYISAMCHK(1)
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