On Thu, Nov 9, 2017 at 3:55 AM, amul sul <sula...@gmail.com> wrote: > It took me a little while to understand this calculation. You have moved this > code from tbm_create(), but I think you should move the following > comment as well:
I made an adjustment that I hope will address your concern here, made a few other adjustments, and committed this. One point of concern that wasn't entirely addressed in the above discussion is the accuracy of this formula: + lossy_pages = Max(0, heap_pages - maxentries / 2); When I first looked at Dilip's test results, I thought maybe this formula was way off. But on closer study, the formula does a decent (not fantastic) job of estimating the number of exact pages. The fact that the number of lossy pages is off is just because the Mackert and Lohman formula is overestimating how many pages are fetched. Now, in Dilip's results, this formula more often than not - but not invariably - predicted more exact pages than we actually got. So possibly instead of maxentries / 2 we could subtract maxentries or some other multiple of maxentries. I don't know what's actually best here, but I think there's a strong argument that this is an improvement as it stands, and we can adjust it later if it becomes clear what would be better. -- Robert Haas EnterpriseDB: http://www.enterprisedb.com The Enterprise PostgreSQL Company -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
