postgres.git / summary / log / commit / refs
commit c06bf14e87c0fe9cda937a5de3ec2a698b249554
Author: David Rowley <drowley@postgresql.org>
Date: Fri Aug 21 00:26:34 2026 +0000
Attempt to stabilize plan of self-join test in tidscan.sql
The test that checks the expected plan for this self-join test has been
known to have failed in the past due to badly timed VACUUMs causing
small variations in row estimates on one of the tables, resulting in a
swapped join order. Currently, failures have only been seen in v14, and
seemingly due to 74388a1ac and 4496020e6 the failures have not been seen
in more recent versions.
Here we shrink down the number of matching rows on one side of the join
to make the alternative join order's costs more expensive relative to
the cheapest join order. Previously the alternative order had the same
cost.
We do this in all supported versions to reduce the chances of future
changes reintroducing stability issues with these queries.
Reported-by: Alexander Lakhin <exclusion@gmail.com>
Author: David Rowley <dgrowleyml@gmail.com>
Discussion: https://postgr.es/m/f5d1f4c2-6224-4797-be17-c86e77f96c9c@gmail.com
Backpatch-through: 14
src/test/regress/expected/tidscan.out | 22 ++++++++++++++--------
src/test/regress/sql/tidscan.sql | 16 ++++++++++++----
2 files changed, 26 insertions(+), 12 deletions(-)
diff --git a/src/test/regress/expected/tidscan.out b/src/test/regress/expected/tidscan.out
index e823bc91c57..52250e09c95 100644
--- a/src/test/regress/expected/tidscan.out
+++ b/src/test/regress/expected/tidscan.out
@@ -237,7 +237,8 @@ ROLLBACK;
-- (these plans don't use TID scans, but this still seems like an
-- appropriate place for these tests)
EXPLAIN (COSTS OFF)
-SELECT count(*) FROM tenk1 t1 JOIN tenk1 t2 ON t1.ctid = t2.ctid;
+SELECT count(*) FROM tenk1 t1 JOIN tenk1 t2 ON t1.ctid = t2.ctid
+WHERE t2.ten = 0;
QUERY PLAN
----------------------------------------
Aggregate
@@ -246,17 +247,20 @@ SELECT count(*) FROM tenk1 t1 JOIN tenk1 t2 ON t1.ctid = t2.ctid;
-> Seq Scan on tenk1 t1
-> Hash
-> Seq Scan on tenk1 t2
-(6 rows)
+ Filter: (ten = 0)
+(7 rows)
-SELECT count(*) FROM tenk1 t1 JOIN tenk1 t2 ON t1.ctid = t2.ctid;
+SELECT count(*) FROM tenk1 t1 JOIN tenk1 t2 ON t1.ctid = t2.ctid
+WHERE t2.ten = 0;
count
-------
- 10000
+ 1000
(1 row)
SET enable_hashjoin TO off;
EXPLAIN (COSTS OFF)
-SELECT count(*) FROM tenk1 t1 JOIN tenk1 t2 ON t1.ctid = t2.ctid;
+SELECT count(*) FROM tenk1 t1 JOIN tenk1 t2 ON t1.ctid = t2.ctid
+WHERE t2.ten = 0;
QUERY PLAN
-----------------------------------------
Aggregate
@@ -268,12 +272,14 @@ SELECT count(*) FROM tenk1 t1 JOIN tenk1 t2 ON t1.ctid = t2.ctid;
-> Sort
Sort Key: t2.ctid
-> Seq Scan on tenk1 t2
-(9 rows)
+ Filter: (ten = 0)
+(10 rows)
-SELECT count(*) FROM tenk1 t1 JOIN tenk1 t2 ON t1.ctid = t2.ctid;
+SELECT count(*) FROM tenk1 t1 JOIN tenk1 t2 ON t1.ctid = t2.ctid
+WHERE t2.ten = 0;
count
-------
- 10000
+ 1000
(1 row)
RESET enable_hashjoin;
diff --git a/src/test/regress/sql/tidscan.sql b/src/test/regress/sql/tidscan.sql
index 1b82d5f1a53..fcea11c027a 100644
--- a/src/test/regress/sql/tidscan.sql
+++ b/src/test/regress/sql/tidscan.sql
@@ -86,12 +86,20 @@ ROLLBACK;
-- (these plans don't use TID scans, but this still seems like an
-- appropriate place for these tests)
EXPLAIN (COSTS OFF)
-SELECT count(*) FROM tenk1 t1 JOIN tenk1 t2 ON t1.ctid = t2.ctid;
-SELECT count(*) FROM tenk1 t1 JOIN tenk1 t2 ON t1.ctid = t2.ctid;
+SELECT count(*) FROM tenk1 t1 JOIN tenk1 t2 ON t1.ctid = t2.ctid
+WHERE t2.ten = 0;
+
+SELECT count(*) FROM tenk1 t1 JOIN tenk1 t2 ON t1.ctid = t2.ctid
+WHERE t2.ten = 0;
+
SET enable_hashjoin TO off;
+
EXPLAIN (COSTS OFF)
-SELECT count(*) FROM tenk1 t1 JOIN tenk1 t2 ON t1.ctid = t2.ctid;
-SELECT count(*) FROM tenk1 t1 JOIN tenk1 t2 ON t1.ctid = t2.ctid;
+SELECT count(*) FROM tenk1 t1 JOIN tenk1 t2 ON t1.ctid = t2.ctid
+WHERE t2.ten = 0;
+
+SELECT count(*) FROM tenk1 t1 JOIN tenk1 t2 ON t1.ctid = t2.ctid
+WHERE t2.ten = 0;
RESET enable_hashjoin;
-- check predicate lock on CTID
[parent: ca99016f939d]