Matthew Wakeling <matthew 'at' flymine.org> writes: > On Thu, 4 Sep 2008, Tom Lane wrote: >> Ultimately the only way that we could get the right answer would be if >> the planner realized that the required rows are concentrated at the end >> of the table instead of being randomly scattered. This isn't something >> that is considered at all right now in seqscan cost estimates. I'm not >> sure offhand whether the existing correlation stats would be of use for >> it, or whether we'd have to get ANALYZE to gather additional data. > > Using the correlation would help, I think, although it may not be the > best solution possible. At least, if the correlation is zero, you > could behave as currently, and if the correlation is 1, then you know > (from the histogram) where in the table the values are. It seems to me that if the correlation is 0.99[1], and you're looking for less than 1% of rows, the expected rows may be at the beginning or at the end of the heap? Ref: [1] or even 1, as ANALYZE doesn't sample all the rows? -- Guillaume Cottenceau, MNC Mobile News Channel SA, an Alcatel-Lucent Company Av. de la Gare 10, 1003 Lausanne, Switzerland - direct +41 21 317 50 36
