"Steinar H. Gunderson" <sgunderson@xxxxxxxxxxx> writes: > On Fri, Jan 20, 2006 at 06:52:37PM -0500, Tom Lane wrote: >> It's also worth considering that the entire approach is a heuristic, >> really --- getting the furthest-apart pair of seeds doesn't guarantee >> an optimal split as far as I can see. Maybe there's some totally >> different way to do it. > For those of us who don't know what tsearch2/gist is trying to accomplish > here, could you provide some pointers? :-) Well, we're trying to split an index page that's gotten full into two index pages, preferably with approximately equal numbers of items in each new page (this isn't a hard requirement though). I think the true figure of merit for a split is how often will subsequent searches have to descend into *both* of the resulting pages as opposed to just one --- the less often that is true, the better. I'm not very clear on what tsearch2 is doing with these bitmaps, but it looks like an upper page's downlink has the union (bitwise OR) of the one-bits in the values on the lower page, and you have to visit the lower page if this union has a nonempty intersection with the set you are looking for. If that's correct, what you really want is to divide the values so that the unions of the two sets have minimal overlap ... which seems to me to have little to do with what the code does at present. Teodor, Oleg, can you clarify what's needed here? regards, tom lane
