---- Fully-indexed Fast Query appearing in the MySQL Slow Log
brotherhood_of_LAN - 3:53 pm on Mar 20, 2011 (gmt 0)
what would that possibly reveal?
MyISAM tables use the query cache whenever possible which is faster than retrieving the result set by actually performing the query.
MyISAM also uses a key buffer [dev.mysql.com] to store frequently accessed index blocks.
An insert on your tables would mean the query has to be performed... because results in query cache may be obsolete after the insert.
I'm not sure why it's being logged as slow since you say the query is being performed quickly...it's worth looking into (as you're doing) even if the queries are of adequate speed.
If it's your own server and you can allocate memory, it may be worth checking out the LOAD INDEX INTO CACHE [dev.mysql.com] route, to store your indexes fully into memory.
Your EXPLAIN pretty much says the indexes are being used so I can only guess that disk IO is the issue.
It's also worth considering that MyISAM tables are locked when INSERTs are happening. So sometimes your SELECTs may have to wait until the INSERTs are done, which can be a problem if the INSERTs are slow.