Manfred Koizar <[EMAIL PROTECTED]> writes: > This patch implements the new tuple sampling method as discussed on > -hackers and -performance a few weeks ago.
Applied with minor editorializations. AFAICS get_next_S() needs to be called with the number of tuples already processed, which means you were off-by-one --- this surely makes only a trivial difference in the probabilities, but if we are going to use Vitter's algorithm then we may as well get it right. Also, I took out the TupleCount typedef and went back to using doubles for the tuple counts; this is more consistent with the coding style used elsewhere, and I really doubt that it's any slower. (The datatype conversions induced inside get_next_S are likely to outweigh any savings from counting by ints, on most modern hardware.) Plus the justification for assuming it couldn't overflow seems weak to me; the current limitation to 300000 requested sample rows is very arbitrary and could change anytime. I was initially convinced that your implementation of Knuth's algorithm S was all wet, so now there's a bunch of comments explaining why it's actually correct... regards, tom lane ---------------------------(end of broadcast)--------------------------- TIP 4: Don't 'kill -9' the postmaster