Stephen Frost wrote: -- Start of PGP signed section. > * Bruce Momjian ([email protected]) wrote: > > Thanks, TODO item readded with a clearer description: > > > > * Allow ORDER BY ... LIMIT 1 to select high/low value without sort or > > index using a sequential scan for highest/lowest values > > > > Right now, if no index exists, ORDER BY ... LIMIT 1 requires we sort > > all values to return the high/low value. Instead The idea is to do a > > sequential scan to find the high/low value, thus avoiding the sort. > > Could we take this perhaps a step further and consider things like > 'LIMIT 10' and come up with an approximate point where the trade-off > exists? Actually, thinking about this a minute more perhaps there isn't > even a trade-off to be made... What you're suggesting is basically a > size-of-1 temporary memory structure for the 'sort'. Isn't there > already a memory structure used to perform the sorting though? Could it > be adjusted such that it's of a fixed size when 'LIMIT' is given, as > above? > > Just some thoughts, while I think the specific 'LIMIT 1' case is > probably pretty common I think the 'LIMIT 10' or 'LIMIT 50' (or however > many you want to display on the webpage...) is a pretty common use case > too and it sounds like we could improve those too with this mechanism.
Yes, I think the final optimization will allow >1 values for LIMIT. -- Bruce Momjian | http://candle.pha.pa.us [email protected] | (610) 359-1001 + If your life is a hard drive, | 13 Roberts Road + Christ can be your backup. | Newtown Square, Pennsylvania 19073 ---------------------------(end of broadcast)--------------------------- TIP 5: Have you checked our extensive FAQ? http://www.postgresql.org/docs/faq
