• MySQL 5.1 Reference Manual :: 12 SQL Statement Syntax :: 12.4 Database Administration Statements :: 12.4.2 Table Maintenance Statements :: 12.4.2.3 CHECK TABLE Syntax

  • 12.4.2.3. CHECK TABLE Syntax

    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 MyISAM, InnoDB, and ARCHIVE tables. Starting with MySQL 5.1.9, CHECK TABLE is also valid for CSV tables, see Section 13.13, “The CSV Storage Engine”. For MyISAM tables, the key statistics are updated as well.

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

    Beginning with MySQL 5.1.27, CHECK TABLE is also supported for partitioned tables. Also beginning with MySQL 5.1.27, you can use ALTER TABLE ... CHECK PARTITION to check one or more partitions; for more information, see Section 12.1.7, “ALTER TABLE Syntax”, and Section 18.3.3, “Maintenance of Partitions”.

    CHECK TABLE returns a result set with the following columns.

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

    Note that 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 you should normally run a repair of the table. See Section 6.6, “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.

    The FOR UPGRADE option checks whether the named tables are compatible with the current version of MySQL. This option was added in MySQL 5.1.7. 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.

    Currently, 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.

    • As of MySQL 5.1.25, 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 2.4.3, “Checking Whether Tables or Indexes Must Be Rebuilt”.

    The other check options that can be given are shown in the following table. These options are passed to the storage engine, which may use them or not. MyISAM uses them; they are ignored for InnoDB tables and views.

    Type Meaning
    QUICK Do not scan the rows to check for incorrect links.
    FAST Check only tables that have not been closed properly.
    CHANGED Check only tables that have been changed since the last check or that have not been closed properly.
    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.
    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.

    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

    In some cases, CHECK TABLE changes the table. This happens if the table is marked as “corrupted” or “not closed properly” but CHECK TABLE does not find any problems in the table. In this case, CHECK TABLE marks the table as okay.

    If a table is corrupted, it is most likely that the problem is 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, simply execute an UPDATE statement to set the column to some value other than 0.

    • If CHECK TABLE finds a problem for an InnoDB table, the server shuts down to prevent error propagation. Details of the error will be written to the error log.