Hi, On 2025-03-18 16:35:29 -0400, Bruce Momjian wrote: > Uh, the random_page_cost = 4 assumes caching, so it is assuming actual > random I/O to be 40x slower, which I doubt is true for SSDs:
Uh, huh: > > https://www.postgresql.org/docs/current/runtime-config-query.html#RUNTIME-CONFIG-QUERY-CONSTANTS > > Random access to mechanical disk storage is normally much more expensive > than four times sequential access. However, a lower default is used > (4.0) because the majority of random accesses to disk, such as indexed > reads, are assumed to be in cache. The default value can be thought of > as modeling random access as 40 times slower than sequential, while > expecting 90% of random reads to be cached. Is that actually a good description of what we assume? I don't know where that 90% is coming from? Briefly skimming through selfuncs.c and costsize.c I don't see anything. The relevant change: commit c1d9df4fa227781b31be44a5a3024865a7f48049 Author: Bruce Momjian <br...@momjian.us> Date: 2012-02-14 16:54:54 -0500 Document random page cost is only 4x seqeuntial, and not 40x. The relevant discussion seems to be: https://postgr.es/m/4F31A05A.1060506%402ndQuadrant.com But I don't see any origin of that number in that thread. I am not sure if I found the correct email for Greg Smith? Greetings, Andres Freund
