This is a cautionary tale - adding indexes is not always helpful or harmless. I recently added an index to the "players" table to optimize a common query, and as a consequence this other query flipped from innocuous to something that takes infinite time.


select p1.player_name,g.score1,g.time1,g.color1,p2.player_name,g.score2,g.time2,g.color2,g.gamename,gmtdate
from players as p1, players as p2, gamerecord g
where (p1.uid = g.player1 and p2.uid = g.player2)
and (p1.is_robot is null and p2.is_robot is null)
order by gmtdate desc limit 50


"players" is a table with 20,000 records, "gamerecord" is a table with 3.5 million records, with "gmtdate" available as an index. The according to "explain", the query used gmtdate as an index, an excellent choice. When I added an index to "is_robot" on the players table, the query flipped to using it, and switched from a brisk report to an infinite slog.

I realize that selecting an index is an imprecise science, and I that can specify what index to use as a hint, but this particular flip was particularly disastrous. It seems odd that the query optimizer would choose to scan a 3.5 million entry table instead of a 20,000 entry table.


--
MySQL General Mailing List
For list archives: http://lists.mysql.com/mysql
To unsubscribe: http://lists.mysql.com/mysql?unsub=listsearcharchive@lassosoft.com