Information about table definitions is stored both in the .frm files, and in the InnoDB data dictionary. If you move .frm files around, or if the server crashes in the middle of a data dictionary operation, these sources of information can become inconsistent.

Problem with CREATE TABLE

A symptom of an out-of-sync data dictionary is that a CREATE TABLE statement fails. If this occurs, look in the server's error log. If the log says that the table already exists inside the InnoDB internal data dictionary, you have an orphaned table inside the InnoDB tablespace files that has no corresponding .frm file. The error message looks like this:

InnoDB: Error: table test/parent already exists in InnoDB internal
InnoDB: data dictionary. Have you deleted the .frm file
InnoDB: and not used DROP TABLE? Have you used DROP DATABASE
InnoDB: for InnoDB tables in MySQL version <= 3.23.43?
InnoDB: See the Restrictions section of the InnoDB manual.
InnoDB: You can drop the orphaned table inside InnoDB by
InnoDB: creating an InnoDB table with the same name in another
InnoDB: database and moving the .frm file to the current database.
InnoDB: Then MySQL thinks the table exists, and DROP TABLE will
InnoDB: succeed.

You can drop the orphaned table by following the instructions given in the error message. If you are still unable to use DROP TABLE successfully, the problem may be due to name completion in the mysql client. To work around this problem, start the mysql client with the --skip-auto-rehash option and try DROP TABLE again. (With name completion on, mysql tries to construct a list of table names, which fails when a problem such as just described exists.)

Problem Opening Table

Another symptom of an out-of-sync data dictionary is that MySQL prints an error that it cannot open a .InnoDB file:

ERROR 1016: Can't open file: 'child2.InnoDB'. (errno: 1)

In the error log you can find a message like this:

InnoDB: Cannot find table test/child2 from the internal data dictionary
InnoDB: of InnoDB though the .frm file for the table exists. Maybe you
InnoDB: have deleted and recreated InnoDB data files but have forgotten
InnoDB: to delete the corresponding .frm files of InnoDB tables?

This means that there is an orphaned .frm file without a corresponding table inside InnoDB. You can drop the orphaned .frm file by deleting it manually.

Problem with Temporary Table

If MySQL crashes in the middle of an ALTER TABLE operation, you may end up with an orphaned temporary table inside the InnoDB tablespace. Using the Table Monitor, you can see listed a table with a name that begins with #sql-. You can perform SQL statements on tables whose name contains the character “#” if you enclose the name within backticks. Thus, you can drop such an orphaned table like any other orphaned table using the method described earlier. To copy or rename a file in the Unix shell, you need to put the file name in double quotation marks if the file name contains “#”.

Problem with Missing Tablespace

With innodb_file_per_table enabled, the following message might occur if the .frm or .ibd files (or both) are missing:

InnoDB: in InnoDB data dictionary has tablespace id N,
InnoDB: but tablespace with that id or name does not exist. Have
InnoDB: you deleted or moved .ibd files?
InnoDB: This may also be a table created with CREATE TEMPORARY TABLE
InnoDB: whose .ibd and .frm files MySQL automatically removed, but the
InnoDB: table still exists in the InnoDB internal data dictionary.

If this occurs, try the following procedure to resolve the problem: