From 862092932c9479b79732f3b441da05453ea5e06d Mon Sep 17 00:00:00 2001 From: Peter Geoghegan Date: Tue, 15 Sep 2026 14:36:47 -0400 Subject: [PATCH] Have read_stream_reset reset pending_read_nblocks. Otherwise, a read of blocks the stream is about to throw away can be started and then waited for. read_stream_reset drains the stream by calling read_stream_next_buffer, whose look-ahead treats the zeroed distance as end of stream, leading to it spuriously performing the pending read. This caused a pathological regression for a nested loop antijoin query when run with the upcoming index prefetching patch series applied. This bug affects all stable branches with the read stream, which was added to Postgres 17 by commit b5a9b18cd0b. No backpatch for now, though, since without index prefetching the added overhead for realistic query plans is negligible. Author: Tomas Vondra Discussion: https://postgr.es/m/CAH2-WzkKY8b42sZSPWJceT5Fn_THk3ZgcaM8ch3Hzw+XvhP0zQ@mail.gmail.com --- src/backend/storage/aio/read_stream.c | 3 +++ 1 file changed, 3 insertions(+) diff --git a/src/backend/storage/aio/read_stream.c b/src/backend/storage/aio/read_stream.c index c865fbc2c9f..fddf6ef441d 100644 --- a/src/backend/storage/aio/read_stream.c +++ b/src/backend/storage/aio/read_stream.c @@ -1444,6 +1444,9 @@ read_stream_reset(ReadStream *stream) stream->readahead_distance = 0; stream->combine_distance = 0; + /* Forget pending reads. */ + stream->pending_read_nblocks = 0; + /* Forget buffered block number and fast path state. */ stream->buffered_blocknum = InvalidBlockNumber; stream->fast_path = false;