MySQL Reference Manual for version 4.0.18.

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7.4.7 How MySQL Counts Open Tables

When you run mysqladmin status, you'll 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 only 6 tables.

MySQL is multi-threaded, so there may be many clients issuing queries for a given simultaneously. To minimise the problem with two client threads having different states on the same file, the table is opened independently by each concurrent thread. This takes some memory but will normally increase performance. With ISAM and MyISAM tables, one extra file descriptor is required for the datafile for each client that has the table open. With these table types, the index file descriptor is shared between all threads.

You can read more about this topic in the next section. See section 7.4.8 How MySQL Opens and Closes Tables.



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