On Wed, Nov 9, 2016 at 6:57 PM, Robert Haas <robertmh...@gmail.com> wrote: > I guess that's possible, but the problem with polyphase merge is that > the increased I/O becomes a pretty significant cost in a hurry.
Not if you have a huge RAID array. :-) Obviously I'm not seriously suggesting that we revise the cap from 500 to 7. We're only concerned about the constant factors here. There is a clearly a need to make some simplifying assumptions. I think that you understand this very well, though. > Maybe another way of putting this is that, while there's clearly a > benefit to having some kind of a cap, it's appropriate to pick a large > value, such as 500. Having no cap at all risks creating many extra > tapes that just waste memory, and also risks an unduly > cache-inefficient final merge. Reigning that in makes sense. > However, we can't reign it in too far or we'll create slow polyphase > merges in case that are reasonably likely to occur in real life. I completely agree with your analysis. -- Peter Geoghegan -- Sent via pgsql-hackers mailing list (firstname.lastname@example.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers