On Mon, 2007-11-05 at 09:52 +0000, Heikki Linnakangas wrote: > Reducing VACUUM time is important, but the real big promise is the > ability to do index-only-scans.
Have you thought about how index-only scans work work? Seems like we need a rough plan for that before we go and build the visibility map, your other notes for which sound very good. I'm thinking that looking in the visibility map will have a cost also, so how will we know whether to bother looking? I'm assuming that we won't want to do that lookup in all cases, since it could easily just add pathlength and contention in the normal OLTP case. Presumably there would be a test in the planner to see if an index-only plan was possible? I'm racking my brain trying to think of a query that will benefit from index-only scans without specifically creating covered indexes. Apart from count(*) queries and RI lookups. I can't see RI lookups being much cheaper with this technique, do you see something there? ISTM that it would make most sense to do it during BitmapIndex scans. Specifically, as an intermediate step between BitmapIndex scan and BitmapHeap scan. That would be fairly likely to be a win in most cases because the bitmaps should compare easily and the amortised cost per row is likely to be very small, even if no heap lookups are avoided. Anyway, just a few initial thoughts. -- Simon Riggs 2ndQuadrant http://www.2ndQuadrant.com ---------------------------(end of broadcast)--------------------------- TIP 2: Don't 'kill -9' the postmaster
