From 158f5819236806b7c9cab323658c231e9371c458 Mon Sep 17 00:00:00 2001 From: Thomas Munro Date: Sun, 7 Apr 2024 14:38:20 +1200 Subject: [PATCH] Fix if/while thinko in read_stream.c edge case. When we determine that a wanted block can't be combined with the current pending read, it's time to start that read to get it out of the way. An "if" in that code path should have been a "while", because it might take more than one go in case of partial reads. This was only broken for smaller ranges, as the more common case of io_combine_limit-sized ranges is handled earlier in the code and knows how to loop, hiding the bug for a while. Discovered while testing large parallel sequential scans of partially cached tables. The ramp-up-and-down block allocator for parallel scans could hit the problem case and skip some blocks near the end that should have been streamed. Defect in commit b5a9b18c. Discussion: https://postgr.es/m/CA%2BhUKG%2Bh8Whpv0YsJqjMVkjYX%2B80fTVc6oi-V%2BzxJvykLpLHYQ%40mail.gmail.com --- src/backend/storage/aio/read_stream.c | 2 +- 1 file changed, 1 insertion(+), 1 deletion(-) diff --git a/src/backend/storage/aio/read_stream.c b/src/backend/storage/aio/read_stream.c index 9a70a81f7ae..f54dacdd914 100644 --- a/src/backend/storage/aio/read_stream.c +++ b/src/backend/storage/aio/read_stream.c @@ -363,7 +363,7 @@ read_stream_look_ahead(ReadStream *stream, bool suppress_advice) } /* We have to start the pending read before we can build another. */ - if (stream->pending_read_nblocks > 0) + while (stream->pending_read_nblocks > 0) { read_stream_start_pending_read(stream, suppress_advice); suppress_advice = false; -- 2.39.5