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MySQL - 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 multithreaded, so it may have many queries on the same table simultaneously. To minimize 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 and one extra file descriptor for the data file. The index file descriptor is shared between all threads.
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