10.4.3.1 How MySQL Opens and Closes Tables (original) (raw)

10.4.3.1 How MySQL Opens and Closes Tables

When you execute a mysqladmin status command, you should see something like this:

Uptime: 426 Running threads: 1 Questions: 11082
Reloads: 1 Open tables: 12

The Open tables value of 12 can be somewhat puzzling if you have fewer than 12 tables.

MySQL is multithreaded, so there may be many clients issuing queries for a given table simultaneously. To minimize the problem with multiple client sessions having different states on the same table, the table is opened independently by each concurrent session. This uses additional memory but normally increases performance. With MyISAM tables, one extra file descriptor is required for the data file for each client that has the table open. (By contrast, the index file descriptor is shared between all sessions.)

The table_open_cache andmax_connections system variables 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. Many operating systems permit you to increase the open-files limit, although the method varies widely from system to system. Consult your operating system documentation to determine whether it is possible to increase the limit and how to do so.

table_open_cache is related to max_connections. For example, for 200 concurrent running connections, specify a table cache size of at least 200 *_`N`_, where_N_ is the maximum number of tables per join in any of the queries which you execute. You must also 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 thetable_open_cache setting. Iftable_open_cache is set too high, MySQL may run out of file descriptors and exhibit symptoms such as refusing connections or failing to perform queries.

Also take into account that the MyISAM storage engine needs two file descriptors for each unique open table. To increase the number of file descriptors available to MySQL, set theopen_files_limit system variable. See Section B.3.2.16, “File Not Found and Similar Errors”.

The cache of open tables is kept at a level oftable_open_cache entries. The server autosizes the cache size at startup. To set the size explicitly, set thetable_open_cache system variable at startup. MySQL may temporarily open more tables than this to execute queries, as described later in this section.

MySQL closes an unused table and removes it 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 thantable_open_cache entries and a table in the cache is no longer being used by any threads.
• When a table-flushing operation occurs. This happens when someone issues a FLUSH TABLES statement or executes amysqladmin flush-tables ormysqladmin refresh command.

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

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

A MyISAM table is opened for each concurrent access. This means the table needs to be opened twice if two threads access the same table or if a thread accesses the table twice in the same query (for example, by joining the table to itself). Each concurrent open requires an entry in the table cache. The first open of anyMyISAM table takes two file descriptors: one for the data file and one for the index file. Each additional use of the table takes only one file descriptor for the data file. The index file descriptor is shared among all threads.

If you are opening a table with the HANDLER_`tblname`_ OPEN statement, a dedicated table object is allocated for the thread. This table object is not shared by other threads and is not closed until the thread calls HANDLER_`tblname`_ CLOSE or the thread terminates. When this happens, the table is put back in the table cache (if the cache is not full). SeeSection 15.2.5, “HANDLER Statement”.

To determine whether your table cache is too small, check theOpened_tables status variable, which indicates the number of table-opening operations since the server started:

mysql> SHOW GLOBAL STATUS LIKE 'Opened_tables';
+---------------+-------+
| Variable_name | Value |
+---------------+-------+
| Opened_tables | 2741  |
+---------------+-------+

If the value is very large or increases rapidly, even when you have not issued many FLUSH TABLES statements, increase thetable_open_cache value at server startup.