Re: index prefetching

On Tue, Aug 26, 2025 at 2:18 AM Tomas Vondra <tomas@vondra.me> wrote:
> Of course, this can happen even with other hit ratios, there's nothing
> special about 50%.

Right, that's what this patch was attacking directly, basically only
giving up when misses are so sparse we can't do anything about it for
an ordered stream:

https://www.postgresql.org/message-id/CA%2BhUKGL2PhFyDoqrHefqasOnaXhSg48t1phs3VM8BAdrZqKZkw%40mail.gmail.com

aio: Improve read_stream.c look-ahead heuristics C

Previously we would reduce the look-ahead distance by one every time we
got a cache hit, which sometimes performed poorly with mixed hit/miss
patterns, especially if it was trapped at one.

Instead, sustain the current distance until we've seen evidence that
there is no window big enough to span the gap between rare IOs.  In
other words, we now use information from a much larger window to
estimate the utility of looking far ahead.