CHECK TABLE SyntaxCHECK TABLE tbl_name[,tbl_name...] [option [option...]] option = QUICK | FAST | MEDIUM | EXTENDED | CHANGED
CHECK TABLE works only on MyISAM and InnoDB tables. On
MyISAM tables it's the same thing as running myisamchk -m
table_name on the table.
If you don't specify any option MEDIUM is used.
Checks the table(s) for errors. For MyISAM tables the key statistics
are updated. The command returns a table with the following columns:
| Column | Value |
| Table | Table name. |
| Op | Always ``check''. |
| Msg_type | One of status, error, info, or warning.
|
| Msg_text | The message. |
Note that you can get many rows of information for each checked table. The
last row will be of Msg_type status and should normally be
OK. If you don't get OK, or Table is already up to
date you should normally run a repair of the table. See section 4.4.6 Using myisamchk for Table Maintenance and Crash Recovery. Table is already up to date means that the table the
given TYPE told MySQL that there wasn't any need to check the
table.
The different check types stand for the following:
| Type | Meaning |
QUICK | Don't scan the rows to check for wrong links. |
FAST | Only check tables which haven't been closed properly. |
CHANGED | Only check tables which have been changed since last check or haven't been closed properly. |
MEDIUM | Scan rows to verify that deleted links are okay. This also calculates a key checksum for the rows and verifies this with a calculated checksum for the keys. |
EXTENDED | Do a full key lookup for all keys for each row. This ensures that the table is 100% consistent, but will take a long time! |
For dynamically sized MyISAM tables a started check will always
do a MEDIUM check. For statically sized rows we skip the row scan
for QUICK and FAST as the rows are very seldom corrupted.
You can combine check options as in:
CHECK TABLE test_table FAST QUICK;
Which would simply do a quick check on the table to see whether it was closed properly.
Note: that in some case CHECK TABLE will change the
table! This happens if the table is marked as 'corrupted' or 'not
closed properly' but CHECK TABLE didn't find any problems in the
table. In this case CHECK TABLE will mark the table as okay.
If a table is corrupted, then it's most likely that the problem is in the indexes and not in the data part. All of the above check types checks the indexes thoroughly and should thus find most errors.
If you just want to check a table that you assume is okay, you should use
no check options or the QUICK option. The latter should be used
when you are in a hurry and can take the very small risk that
QUICK didn't find an error in the datafile. (In most cases
MySQL should find, under normal usage, any error in the data
file. If this happens then the table will be marked as 'corrupted',
in which case the table can't be used until it's repaired.)
FAST and CHANGED are mostly intended to be used from a
script (for example to be executed from cron) if you want to check your
table from time to time. In most cases you FAST is to be prefered
over CHANGED. (The only case when it isn't is when you suspect a
bug you have found a bug in the MyISAM code.)
EXTENDED is only to be used after you have run a normal check but
still get strange errors from a table when MySQL tries to
update a row or find a row by key (this is very unlikely if a
normal check has succeeded!).
Some things reported by CHECK TABLE can't be corrected automatically:
Found row where the auto_increment column has the value 0.
This means that you have in the table a row where the
AUTO_INCREMENT index column contains the value 0.
(It's possible to create a row where the AUTO_INCREMENT column is 0 by
explicitly setting the column to 0 with an UPDATE statement.)
This isn't an error in itself, but could cause trouble if you decide to
dump the table and restore it or do an ALTER TABLE on the
table. In this case the AUTO_INCREMENT column will change value,
according to the rules of AUTO_INCREMENT columns, which could cause
problems such as a duplicate key error.
To get rid of the warning, just execute an UPDATE statement
to set the column to some other value than 0.