On Fri, Nov 18, 2022 at 2:23 PM Tomas Vondra <tomas.vondra@xxxxxxxxxxxxxxxx> wrote: > That is, when comparing costs, we require the cost to be at least 1%, > because we have a cheapest path, and we're checking if it's worth > building another one (which is not free - we have to allocate stuff > etc.). And if the difference is tiny, it's not worth it. > > In this case we have the indexscan for the hash index, with cost > 8971.95, and we're considering to build indexacan path for the btree > index. We haven't built it yet, we only calculate cost 8891.65. But > > 8971.95/8891.65 = 1.009 > > So it's close to 1.01, but just a little bit less. So we conclude it's > not worth building the second path, and we keep the hash index scan. > An excellent explanation, it totally does make sense to me and explains what I felt (i.e., similar costs lead to a kind of equality in choosing the index). Thanks, Luca
