On Tue, Dec 19, 2017 at 2:55 PM, Haisheng Yuan <hy...@pivotal.io> wrote: > > Below is the graph (credit to Heikki) that plots the total estimated cost > of a bitmap heap scan, where table size is 10000 pages, and > random_page_cost=10 and seq_page_cost=1. X axis is the number of pages > fetche. I.e. on the left, no pages are fetched, and on the right end, at > 10000, all pages are fetched. The original formula is in black, the new > formula in red, and the horizontal line, in blue, shows the cost of a Seq > Scan. > [image: Inline image 3] > > > Thoughts? Any better ideas? >
The parabola-shape curve we're getting at present is clearly wrong; approaching a horizontal line as an asymptote seems much better. However, shouldn't the red line level off at some level *above* the blue line rather than *at* the blue line? Reading the index pages isn't free, so a sequential scan should be preferred when we're going to read the whole table anyway. -- Robert Haas EnterpriseDB: http://www.enterprisedb.com The Enterprise PostgreSQL Company
