mysql slow on first query, then fast for related queries

Question

I have been struggling with a problem that only happens when the database has been idle for a period of time for the data queried. The first query will be extremely slow, o

Answer 1

Ttry and compare the output of "vmstat 1" on the linux command line when running the query after a period of time, vs when you re-run it and get results fast. Specifically check the "bi" column (that's the kb read from disk per second).

You may find the operating system is caching the disk blocks in the fast case (and thus a low "bi" figure), but not in the slow case (and hence a large "bi" figure).

You might also find that vmstat shows high/low cpu usage in either case. If it's low when fast, and disk throughput is also low, then your system may still be returning a cached query, even though you've indicated the relevant config value is set to zero. Perhaps check the output of show engine innodb status and SHOW VARIABLES and confirm.

innodb_buffer_pool_size may also be set high (it should be...), which would cache the blocks even before the OS can return them.

You might also find that "key_buffer" is set high - this would cache the keys in the indexes, which could make your select blindingly fast.

Try check the mysql performance blog site for lots of useful info.