> Setting "track_io_timing = on" should measure the time spent doing I/O more accurately.
I see I/O timings after this. It shows that 96.5% of long queries is spent on I/O. If I subtract I/O time from total I get ~1,4 s for 5000 rows, which is SAME for both ranges if I adjust segment borders accordingly (to match ~5000 rows). Only I/O time differs, and differs significantly.
> One problem with measuring read speed that way is that "buffers read" can mean "buffers read from storage" or "buffers read from the file system cache",
I understand, that's why I conducted experiments with drop_caches.
> but you say you observe a difference even after dropping the cache.
No, I say I see NO significant difference (accurate to measurement error) between "with caches" and after dropping caches. And this is explainable, I think. Since I read consequently almost all data from the huge table, no cache can fit this data, thus it cannot influence significantly on results. And whilst the PK index *could* be cached (in theory) I think its data is being displaced from buffers by bulkier JSONB data.
Vlad