"Jason Coene" <[EMAIL PROTECTED]> writes: > A good example, a comments table where users submit TEXT data. A common > query is to find the last 5 comments a user has submitted. The scan, while > using an index, takes a considerable amount of time (> 0.5 sec is about as > good as it gets). Again, it's using an index on the single WHERE clause > (userid = int). The field that's used to ORDER BY (timestamp) is also > indexed.
You mean you are doing SELECT ... WHERE userid = 42 ORDER BY timestamp DESC LIMIT 5; and hoping that separate indexes on userid and timestamp will get the job done? They won't. There are only two possible plans for this, neither very good: select all of user 42's posts and sort them, or scan timewise backwards through *all* posts looking for the last 5 from user 42. If you do this enough to justify a specialized index, I would suggest a two-column index on (userid, timestamp). You will also need to tweak the query, because the planner is not quite smart enough to deduce that such an index is applicable to the given sort order: SELECT ... WHERE userid = 42 ORDER BY userid DESC, timestamp DESC LIMIT 5; This should generate an index-scan-backwards plan that will execute nigh instantaneously, because it will only fetch the rows you really want. You might or might not be able to drop the separate indexes on userid and timestamp, depending on what other queries you might have that need them. But you should be paying attention to what plans you are really getting (see EXPLAIN) rather than just assuming that some indexes chosen at random will do what you need. regards, tom lane ---------------------------(end of broadcast)--------------------------- TIP 2: you can get off all lists at once with the unregister command (send "unregister YourEmailAddressHere" to [EMAIL PROTECTED])