On 5/9/25 14:53, Aleksander Alekseev wrote: > Hi Tomas, > >> While running some benchmarks comparing 17 and 18, I ran into a simple >> workload where 18 throughput drops by ~80%. After pulling my hair for a >> couple hours I realized the change that triggered this is 04bec894a04c, >> which set checksums on by default. Which is very bizarre, because the >> workload is read-only and fits into shared buffers. >> >> [...] >> >> But why would it depend on checksums at all? This read-only test should >> be entirely in-memory, so how come it's affected? > > These are interesting results. > > Just wanted to clarify: did you make sure that all the hint bits were > set before executing the benchmark? > > I'm not claiming that hint bits are necessarily the reason for the > observed behavior but when something is off with presumably read-only > queries this is the first reason that comes to mind. At least we > should make sure hint bits are excluded from the equation. If memory > serves, VACUUM FULL and CHECKPOINT after filling the table and > creating the index should do the trick. >
Good question. I haven't checked that explicitly, but it's a tiny data set (15MB) and I observed this even on long benchmarks with tens of millions of queries. So the hint bits should have been set. Also, I should have mentioned the query does an index-only scan, and the pin/unpin calls are on index pages, not on the heap. regards -- Tomas Vondra
