Re: Finalizing read stream users' flag choices

hi,

On 2025-04-08 12:06:47 -0400, Melanie Plageman wrote:
> And a somewhat related point, with buffered IO, READ_STREAM_SEQUENTIAL
> disables prefetching to encourage OS readahead. I don't know if any
> other users than sequential scan should do this.

Worth adding that prefetches are only issued when io_method==sync and thus
READ_STREAM_SEQUENTIAL only has an effect if io_method==sync.

I suspect we should retire READ_STREAM_SEQUENTIAL in 19 or so:

a) The sequential-ness detection has gotten smarter / more granular, reducing
the need for forcing read_stream's hand.
b) There are plenty cases where READ_STREAM_SEQUENTIAL *hurts* with seqscans
and io_method==sync, e.g. if there are plenty pre-existing buffers in s_b.
c) It doesn't have an effect with io_method != sync

But it'll depend a bit on our experiences.

Greetings,

Andres Freund