5.4.6 Why So Many 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

This can be somewhat perplexing if you only have 6 tables.

MySQL is multi-threaded, so it may have many queries on the same table simultaneously. To minimise the problem with two threads having different states on the same file, the table is opened independently by each concurrent thread. This takes some memory but will normaly increase performance. With ISAM and MyISAM tables this also requires one extra file descriptor for the datafile. 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 5.4.7 How MySQL Opens and Closes Tables.

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