13.7.2.2 CHECK TABLE Syntax

13.7.2.2 CHECK TABLE Syntax
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CHECK TABLE tbl_name [, tbl_name] ... [option] ...

option = {
    FOR UPGRADE
  | QUICK
  | FAST
  | MEDIUM
  | EXTENDED
  | CHANGED
}

CHECK TABLE checks a table or tables for errors. CHECK TABLE works for InnoDB, MyISAM, ARCHIVE, and CSV tables. For MyISAM tables, the key statistics are updated as well.

Before running CHECK TABLE on InnoDB tables, see CHECK TABLE Usage Notes for InnoDB Tables.

To check a table, you must have some privilege for it.

CHECK TABLE can also check views for problems, such as tables that are referenced in the view definition that no longer exist.

CHECK TABLE is supported for partitioned tables, and you can use ALTER TABLE ... CHECK PARTITION to check one or more partitions; for more information, see Section 13.1.7, “ALTER TABLE Syntax”, and Section 19.3.3, “Maintenance of Partitions”.

Output

CHECK TABLE returns a result set with the following columns.

Column	Value
`Table`	The table name
`Op`	Always `check`
`Msg_type`	`status`, `error`, `info`, `note`, or `warning`
`Msg_text`	An informational message

The statement might produce many rows of information for each checked table. The last row has a Msg_type value of status and the Msg_text normally should be OK. If you don't get OK, or Table is already up to date for a MyISAM table, you should normally run a repair of the table. See Section 7.6, “MyISAM Table Maintenance and Crash Recovery”. Table is already up to date means that the storage engine for the table indicated that there was no need to check the table.

Checking Version Compatibility

The FOR UPGRADE option checks whether the named tables are compatible with the current version of MySQL. With FOR UPGRADE, the server checks each table to determine whether there have been any incompatible changes in any of the table's data types or indexes since the table was created. If not, the check succeeds. Otherwise, if there is a possible incompatibility, the server runs a full check on the table (which might take some time). If the full check succeeds, the server marks the table's .frm file with the current MySQL version number. Marking the .frm file ensures that further checks for the table with the same version of the server will be fast.

Incompatibilities might occur because the storage format for a data type has changed or because its sort order has changed. Our aim is to avoid these changes, but occasionally they are necessary to correct problems that would be worse than an incompatibility between releases.

FOR UPGRADE discovers these incompatibilities:

The indexing order for end-space in TEXT columns for InnoDB and MyISAM tables changed between MySQL 4.1 and 5.0.
The storage method of the new DECIMAL data type changed between MySQL 5.0.3 and 5.0.5.
If your table was created by a different version of the MySQL server than the one you are currently running, FOR UPGRADE indicates that the table has an .frm file with an incompatible version. In this case, the result set returned by CHECK TABLE contains a line with a Msg_type value of error and a Msg_text value of Table upgrade required. Please do "REPAIR TABLE `tbl_name`" to fix it!
Changes are sometimes made to character sets or collations that require table indexes to be rebuilt. For details about these changes and when FOR UPGRADE detects them, see Section 2.11.3, “Checking Whether Tables or Indexes Must Be Rebuilt”.

Checking Data Consistency

The following table shows the other check options that can be given. These options are passed to the storage engine, which may use them or not.

Type	Meaning
`QUICK`	Do not scan the rows to check for incorrect links. Applies to `InnoDB` and `MyISAM` tables and views.
`FAST`	Check only tables that have not been closed properly. Applies only to `MyISAM` tables and views; ignored for `InnoDB`.
`CHANGED`	Check only tables that have been changed since the last check or that have not been closed properly. Applies only to `MyISAM` tables and views; ignored for `InnoDB`.
`MEDIUM`	Scan rows to verify that deleted links are valid. This also calculates a key checksum for the rows and verifies this with a calculated checksum for the keys. Applies only to `MyISAM` tables and views; ignored for `InnoDB`.
`EXTENDED`	Do a full key lookup for all keys for each row. This ensures that the table is 100% consistent, but takes a long time. Applies only to `MyISAM` tables and views; ignored for `InnoDB`.

If none of the options QUICK, MEDIUM, or EXTENDED are specified, the default check type for dynamic-format MyISAM tables is MEDIUM. This has the same result as running myisamchk --medium-check tbl_name on the table. The default check type also is MEDIUM for static-format MyISAM tables, unless CHANGED or FAST is specified. In that case, the default is QUICK. The row scan is skipped for CHANGED and FAST because the rows are very seldom corrupted.

You can combine check options, as in the following example that does a quick check on the table to determine whether it was closed properly:

CHECK TABLE test_table FAST QUICK;

Note

If CHECK TABLE finds no problems with a table that is marked as “corrupted” or “not closed properly”, CHECK TABLE may remove the mark.

If a table is corrupted, the problem is most likely in the indexes and not in the data part. All of the preceding check types check 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 does not find an error in the data file. (In most cases, under normal usage, MySQL should find any error in the data file. If this happens, the table is marked as “corrupted” and cannot be used until it is 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 tables from time to time. In most cases, FAST is to be preferred over CHANGED. (The only case when it is not preferred is when you suspect that you have found a bug in the MyISAM code.)

EXTENDED is to be used only 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.

Use of CHECK TABLE ... EXTENDED might influence the execution plan generated by the query optimizer.

Some problems reported by CHECK TABLE cannot be corrected automatically:

Found row where the auto_increment column has the value 0.
This means that you have a row in the table where the AUTO_INCREMENT index column contains the value 0. (It is possible to create a row where the AUTO_INCREMENT column is 0 by explicitly setting the column to 0 with an UPDATE statement.)
This is not 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 changes 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, execute an UPDATE statement to set the column to some value other than 0.

CHECK TABLE Usage Notes for InnoDB Tables

The following notes apply to InnoDB tables:

If CHECK TABLE encounters a corrupt page, the server exits to prevent error propagation (Bug #10132). If the corruption occurs in a secondary index but table data is readable, running CHECK TABLE can still cause a server exit.
If CHECK TABLE encounters a corrupted DB_TRX_ID or DB_ROLL_PTR field in a clustered index, CHECK TABLE can cause InnoDB to access an invalid undo log record, resulting in an MVCC-related server exit.
If CHECK TABLE encounters errors in InnoDB tables or indexes, it reports an error, and usually marks the index and sometimes marks the table as corrupted, preventing further use of the index or table. Such errors include an incorrect number of entries in a secondary index or incorrect links.
If CHECK TABLE finds an incorrect number of entries in a secondary index, it reports an error but does not cause a server exit or prevent access to the file.
CHECK TABLE surveys the index page structure, then surveys each key entry. It does not validate the key pointer to a clustered record or follow the path for BLOB pointers.
When an InnoDB table is stored in its own .ibd file, the first 3 pages of the .ibd file contain header information rather than table or index data. The CHECK TABLE statement does not detect inconsistencies that affect only the header data. To verify the entire contents of an InnoDB .ibd file, use the innochecksum command.
When running CHECK TABLE on large InnoDB tables, other threads may be blocked during CHECK TABLE execution. To avoid timeouts, the semaphore wait threshold (600 seconds) is extended by 2 hours (7200 seconds) for CHECK TABLE operations. If InnoDB detects semaphore waits of 240 seconds or more it starts printing InnoDB monitor output to the error log. If a lock request extends beyond the semaphore wait threshold, InnoDB aborts the process. To avoid the possibility of a semaphore wait timeout entirely, you can run CHECK TABLE QUICK instead of CHECK TABLE.

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