Timothy Garnett <tgarnett@xxxxxxxxxxx> wrote: >>> -> Index Scan Backward using >>> index_customs_records_on_month_and_bl_number on customs_records >>> (cost=0.00..78426750.74 rows=48623 width=908) (actual >>> time=171344.182..3858893.588 rows=100 loops=1) We've seen a lot of those lately -- Index Scan Backward performing far worse than alternatives. One part of it is that disk sectors are arranged for optimal performance on forward scans; but I don't think we've properly accounted for the higher cost of moving backward through our btree indexes, either. To quote from the README for the btree AM: | A backwards scan has one additional bit of complexity: after | following the left-link we must account for the possibility that | the left sibling page got split before we could read it. So, we | have to move right until we find a page whose right-link matches | the page we came from. (Actually, it's even harder than that; see | deletion discussion below.) I'm wondering whether the planner should have some multiplier or other adjustment to attempt to approximate the known higher cost of backward scans. -Kevin -- Sent via pgsql-performance mailing list (pgsql-performance@xxxxxxxxxxxxxx) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-performance
