In the last episode (Aug 26), Dave Dyer said:
> 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.
> 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.
Can you post the EXPLAIN EXTENDED output for your before and after queries?
also, have you recently run an ANALYZE TABLE on the tables?