pgsql: Improve read_stream.c's fast path.

Improve read_stream.c's fast path.

The "fast path" for well cached scans that don't do any I/O was
accidentally coded in a way that could only be triggered by pg_prewarm's
usage pattern, which starts out with a higher distance because of the
flags it passes in. We want it to work for streaming sequential scans
too, once that patch is committed. Adjust.

Reviewed-by: Melanie Plageman <melanieplageman(at)gmail(dot)com>
Discussion: https://postgr.es/m/CA%2BhUKGKXZALJ%3D6aArUsXRJzBm%3Dqvc4AWp7%3DiJNXJQqpbRLnD_w%40mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/aa1e8c206454d666ab600050022aef2c3c390f69

Modified Files
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src/backend/storage/aio/read_stream.c | 75 +++++++++++++++--------------------
1 file changed, 31 insertions(+), 44 deletions(-)