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Subject: BUG #19687: ALTER SEQUENCE provokes error XX001 could not read blocks
To: pgsql-bugs@lists.postgresql.org
From: PG Bug reporting form <noreply@postgresql.org>
Cc: exclusion@gmail.com
Reply-To: exclusion@gmail.com, pgsql-bugs@lists.postgresql.org
Date: Mon, 14 Sep 2026 05:00:01 +0000
Message-ID: <19687-dd094472b0c48afb@postgresql.org>
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 <https://www.postgresql.org/message-id/19687-dd094472b0c48afb%40postgresql.org>
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The following bug has been logged on the website:

Bug reference:      19687
Logged by:          Alexander Lakhin
Email address:      exclusion@gmail.com
PostgreSQL version: 19beta3
Operating system:   Ubuntu 24.04
Description:       =20

The following script:
psql -c "CREATE SEQUENCE s AS bigint MAXVALUE 1000000;"
for ((i=3D1; i<=3D100; i++)); do
  echo "iteration $i"
  for ((i=3D1; i<=3D100; i++)); do echo "SELECT * FROM s;"; done | psql
>/dev/null &
  for ((i=3D1; i<=3D100; i++)); do echo "ALTER SEQUENCE s AS int;"; done | =
psql
>/dev/null
  grep -A1 "ERROR:  could not read block" server.log && break;
  wait
done

triggers:
iteration 3
ERROR:  could not read blocks 0..0 in file "base/16384/16597": read only 0
of 8192 bytes
2026-09-14 07:53:16.554 EEST [2078499:4] psql XX001 ERROR:  could not read
blocks 0..0 in file "base/16384/16597": read only 0 of 8192 bytes
2026-09-14 07:53:16.554 EEST [2078499:5] psql XX001 STATEMENT:  SELECT *
FROM s;

This error is easily reproduced with io_workers, but it can be reproduced
starting from 3d79013b9, with increased number of iterations and SELECT
clients.