5.4.7 How MySQL Opens and Closes Tables

table_cache, max_connections, and max_tmp_tables affect the maximum number of files the server keeps open. If you increase one or both of these values, you may run up against a limit imposed by your operating system on the per-process number of open file descriptors. However, you can increase the limit on many systems. Consult your OS documentation to find out how to do this, because the method for changing the limit varies widely from system to system.

table_cache is related to max_connections. For example, for 200 concurrent running connections, you should have a table cache of at least 200 * n, where n is the maximum number of tables in a join. You also need to reserve some extra file descriptors for temporary tables and files.

Make sure that your operating system can handle the number of open file descriptors implied by the table_cache setting. If table_cache is set too high, MySQL may run out of file descriptors and refuse connections, fail to perform queries, and be very unreliable. You also have to take into account that the MyISAM storage engine needs two file descriptors for each unique open table. You can in increase the number of file descriptors available for MySQL with the --open-files-limit=# startup option. See section A.2.17 File Not Found.

The cache of open tables will be kept at a level of table_cache entries. The default value is 64; this can be changed with the -O table_cache=# option to mysqld). Note that MySQL may temporarily open even more tables to be able to execute queries.

A not used table is closed and removed from the table cache under the following circumstances:

• When the cache is full and a thread tries to open a table that is not in the cache.
• When the cache contains more than table_cache entries and a thread is no longer using a table.
• When someone executes mysqladmin refresh or mysqladmin flush-tables.
• When someone executes a FLUSH TABLES statement.

When the table cache fills up, the server uses the following procedure to locate a cache entry to use:

• Tables that are not currently in use are released, in least-recently-used order.
• If the cache is full and no tables can be released, but a new table needs to be opened, the cache is temporarily extended as necessary.
• If the cache is in a temporarily extended state and a table goes from in-use to not-in-use state, the table is closed and released from the cache.

A table is opened for each concurrent access. This means that if you have two threads accessing the same table or access the table twice in the same query (with AS) the table needs to be opened twice. The first open of any table takes two file descriptors; each additional use of the table takes only one file descriptor. The extra descriptor for the first open is used for the index file; this descriptor is shared among all threads.

If you are opening a table with the HANDLER table_name OPEN statement, a dedicated table object is allocated for the thread. This table object is not shared by other threads an will not be closed until the thread calls HANDLER table_name CLOSE or the thread dies. See section 6.4.2 HANDLER Syntax. When this happens, the table is put back in the table cache (if it isn't full).

You can check if your table cache is too small by checking the mysqld variable Opened_tables. If this is quite big, even if you haven't done a lot of FLUSH TABLES, you should increase your table cache. See section 4.5.7.3 SHOW STATUS.

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