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 7.4.8 How MySQL Opens and Closes Tables
 ---------------------------------------
 
 The `table_cache', `max_connections', and `max_tmp_tables' system
 variables affect the maximum number of files the server keeps open.  If
 you increase one or more 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 allow 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_cache' is related to `max_connections'.  For example, for 200
 concurrent running connections, you should have a table cache size 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 increase the
 number of file descriptors available for MySQL with the
 `--open-files-limit' startup option to `mysqld_safe'.   Not enough
 file handles.
 
 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
 `--table_cache' option to `mysqld'.  Note that MySQL may temporarily
 open even more tables to be able to execute queries.
 
 An unused 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 a table flushing operation occurs. This happens when someone
      issues a `FLUSH TABLES' statement or executes a `mysqladmin
      flush-tables' or `mysqladmin refresh' command.
 
 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 a new table needs to 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 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 any 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 TBL_NAME 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 TBL_NAME CLOSE' or the thread terminates.  When
 this happens, the table is put back in the table cache (if the cache
 isn't full).   `HANDLER' HANDLER.
 
 You can determine whether your table cache is too small by checking the
 `mysqld' status variable `Opened_tables':
 
      mysql> SHOW STATUS LIKE 'Opened_tables';
      +---------------+-------+
      | Variable_name | Value |
      +---------------+-------+
      | Opened_tables | 2741  |
      +---------------+-------+
 
 If the value is quite big, even when you haven't issued a lot of `FLUSH
 TABLES' statements, you should increase your table cache size.  See
  Server system variables and  Server status variables.
 

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