Tom Lane wrote: > Bruce Momjian <[EMAIL PROTECTED]> writes: > > Do we know in the optimizer whether we will be needing cheapest startup > > or not? > > No. Higher levels might want either. > > > Is the middle one kept because the optimizer has to mix the startup plus > > some percentage of the total cost for queries using LIMIT? > > Right. There are potentially some ranges of LIMIT for which it could > win, I believe. Maybe with some math you could prove there is no range > in which the other two don't dominate it, but I suspect the extra logic > would slow down add_path more than it's worth.
What if we take the total cost and divide it by the number of rows returned --- then we have a per-row cost for each plan. Then we subtract the two, and that difference compared to the difference in startup costs tell us how many rows could potentially use this plan. -- 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 6: Have you searched our list archives? http://archives.postgresql.org
