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From: Matthias van de Meent <[email protected]>
To: PostgreSQL Hackers <[email protected]>
Subject: Instability in test/regress/sql/portals.sql
Date: Wed, 11 Mar 2026 15:11:36 +0100
Message-ID: <CAEze2Wi0hg46bcvLQrjid6Kvbb+NSR1N5OKbDNfHy2J=JutJ=Q@mail.gmail.com> (raw)

Hi,

Internally at Databricks we've seen rare regression failures in the
portals.sql test, where the regression diff looks something like the
one attached in data_attachments_ed2b37649a9393b5.diffs.

It seems like this was caused by synchronized seqscans, which caused
the foo25ns cursor to start its seqscan not at the start of the table,
but instead with an offset into the table. This changed the output,
because that relied on the seqscan starting at the first page of the
table.

To stabilize this test, let's add SET synchronize_seqscans = off, as attached.

Kind regards,

Matthias van de Meent
Databricks (https://www.databricks.com)


Attachments:

  [application/octet-stream] data_attachments_ed2b37649a9393b5.diffs (2.9K, 2-data_attachments_ed2b37649a9393b5.diffs)
  download

  [application/octet-stream] v1-0001-Stabilize-syncscan-issue-in-pg_regress-portals.ou.patch (1.4K, 3-v1-0001-Stabilize-syncscan-issue-in-pg_regress-portals.ou.patch)
  download | inline diff:
From 39e7a810c13f64660fc23a027a4ec314a343f36d Mon Sep 17 00:00:00 2001
From: Matthias van de Meent <[email protected]>
Date: Wed, 11 Mar 2026 13:45:28 +0100
Subject: [PATCH v1] Stabilize syncscan issue in pg_regress' portals.out

---
 src/test/regress/expected/portals.out | 2 ++
 src/test/regress/sql/portals.sql      | 3 +++
 2 files changed, 5 insertions(+)

diff --git a/src/test/regress/expected/portals.out b/src/test/regress/expected/portals.out
index 06726ed4ab7..38506abe0df 100644
--- a/src/test/regress/expected/portals.out
+++ b/src/test/regress/expected/portals.out
@@ -1,6 +1,7 @@
 --
 -- Cursor regression tests
 --
+SET synchronize_seqscans = off;
 BEGIN;
 DECLARE foo1 SCROLL CURSOR FOR SELECT * FROM tenk1 ORDER BY unique2;
 DECLARE foo2 SCROLL CURSOR FOR SELECT * FROM tenk2;
@@ -1561,3 +1562,4 @@ fetch all in held_portal;
 (1 row)
 
 reset default_toast_compression;
+reset synchronize_seqscans;
diff --git a/src/test/regress/sql/portals.sql b/src/test/regress/sql/portals.sql
index fc4cccb96c0..995fef597bd 100644
--- a/src/test/regress/sql/portals.sql
+++ b/src/test/regress/sql/portals.sql
@@ -2,6 +2,8 @@
 -- Cursor regression tests
 --
 
+SET synchronize_seqscans = off;
+
 BEGIN;
 
 DECLARE foo1 SCROLL CURSOR FOR SELECT * FROM tenk1 ORDER BY unique2;
@@ -605,3 +607,4 @@ drop table toasted_data;
 fetch all in held_portal;
 
 reset default_toast_compression;
+reset synchronize_seqscans;
-- 
2.50.1 (Apple Git-155)

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