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.
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