Subject: BUG #19531: Inconsistent Error Messages for the Same SQL Query

From: PG Bug reporting form <[email-masked]>
Date: Wed, 24 Jun 2026 07:20:28 +0000

The following bug has been logged on the website:

Bug reference:      19531
Logged by:          Wang Hao
Email address:      [email-masked]
PostgreSQL version: 18.4
Operating system:   Ubuntu22.04
Description:        

Inconsistent Error Messages for the Same SQL Query in PostgreSQL

Environment：
- PostgreSQL version: 18.4
- Operating System: Ubuntu 22.04

Context：
My application relies on the error messages returned by SQL statement
execution to perform subsequent operations. During development, I discovered
that the same SQL query can return different error messages . I'm not sure
if this is a bug, but it does affect my application's ability to reliably
handle errors.

Steps to Reproduce:
Example 1: Triggering different errors by controlling the execution plan
test=# CREATE TABLE t (c1 text DEFAULT '', c2 smallint DEFAULT 0);
CREATE TABLE
test=# INSERT INTO t (c1, c2) VALUES ('jkwdot0re', 0);
INSERT 0 1
test=# INSERT INTO t (c1, c2) VALUES ('', -1000);
INSERT 0 1
test=# SELECT c2 / c2 AS e0, c2 * c2 AS e1 FROM t GROUP BY c1, c2;
ERROR:  division by zero
test=# explain(verbose) SELECT c2 / c2 AS e0, c2 * c2 AS e1 FROM t GROUP BY
c1, c2;
                            QUERY PLAN
-------------------------------------------------------------------
 HashAggregate  (cost=29.65..32.65 rows=200 width=38)
   Output: (c2 / c2), (c2 * c2), c1, c2
   Group Key: t.c1, t.c2
   ->  Seq Scan on public.t  (cost=0.00..23.10 rows=1310 width=34)
         Output: c1, c2
(5 rows)

test=# SET enable_hashagg = off;
SET
test=# SELECT c2 / c2 AS e0, c2 * c2 AS e1 FROM t GROUP BY c1, c2;
ERROR:  smallint out of range
test=# explain(verbose) SELECT c2 / c2 AS e0, c2 * c2 AS e1 FROM t GROUP BY
c1, c2;
                               QUERY PLAN
-------------------------------------------------------------------------
 Group  (cost=90.93..101.75 rows=200 width=38)
   Output: (c2 / c2), (c2 * c2), c1, c2
   Group Key: t.c1, t.c2
   ->  Sort  (cost=90.93..94.20 rows=1310 width=34)
         Output: c1, c2
         Sort Key: t.c1, t.c2
         ->  Seq Scan on public.t  (cost=0.00..23.10 rows=1310 width=34)
               Output: c1, c2
(8 rows)

Example 2:Triggering different errors due to statistics changes
test=# CREATE TABLE t (c1 text DEFAULT '', c2 smallint DEFAULT 0);
CREATE TABLE
test=# INSERT INTO t (c1, c2) VALUES ('jkwdot0re', 0);
INSERT 0 1
test=# INSERT INTO t (c1, c2) VALUES ('', -1000);
INSERT 0 1
test=# SELECT c2 / c2 AS e0, c2 * c2 AS e1 FROM t GROUP BY c1, c2;
ERROR:  division by zero
test=# explain(verbose) SELECT c2 / c2 AS e0, c2 * c2 AS e1 FROM t GROUP BY
c1, c2;
                            QUERY PLAN
-------------------------------------------------------------------
 HashAggregate  (cost=29.65..32.65 rows=200 width=38)
   Output: (c2 / c2), (c2 * c2), c1, c2
   Group Key: t.c1, t.c2
   ->  Seq Scan on public.t  (cost=0.00..23.10 rows=1310 width=34)
         Output: c1, c2
(5 rows)

test=# ANALYZE t;
ANALYZE
test=# SELECT c2 / c2 AS e0, c2 * c2 AS e1 FROM t GROUP BY c1, c2;
ERROR:  smallint out of range
test=# explain(verbose) SELECT c2 / c2 AS e0, c2 * c2 AS e1 FROM t GROUP BY
c1, c2;
                             QUERY PLAN
--------------------------------------------------------------------
 Group  (cost=1.03..1.05 rows=2 width=11)
   Output: (c2 / c2), (c2 * c2), c1, c2
   Group Key: t.c1, t.c2
   ->  Sort  (cost=1.03..1.03 rows=2 width=7)
         Output: c1, c2
         Sort Key: t.c1, t.c2
         ->  Seq Scan on public.t  (cost=0.00..1.02 rows=2 width=7)
               Output: c1, c2
(8 rows)

Observation:
The same SQL query SELECT c2 / c2 AS e0, c2 * c2 AS e1 FROM t GROUP BY c1,
c2; executed against the same data returns two different error messages:
- ERROR: division by zero
- ERROR: smallint out of range
The query contains two expressions that can each trigger an error on the
same data:
- c2 / c2: division by zero when c2 = 0
- c2 * c2: smallint overflow when c2 = -1000 (since (-1000)² = 1,000,000
exceeds the smallint range)
In Example 1, changing enable_hashagg switches the execution plan from
HashAggregate to Group.In Example 2, running ANALYZE updates table
statistics

I'm not certain whether this qualifies as a bug, but it does create a real
problem for applications that depend on consistent error reporting from the
database.Thanks for reading my report!

---

From: Laurenz Albe <[email-masked]>
Date: Wed, 24 Jun 2026 10:19:52 +0200

> if this is a bug, but it does affect my application's ability to reliably

I would say that this is not a bug.

Your statement contains two errors and it depends on the execution plan
which of the two errors you are hitting first.  If your program reacts
differently based on the exact error, both reactions would be correct,
because both errors are present in the query.

I would compare the situation to an INSERT statement into a table with
two constraints.  Depending on which data are present in the database,
you could hit a constraint violation error for either constraint.

Yours,
Laurenz Albe

---

From: Francisco Olarte <[email-masked]>
Date: Wed, 24 Jun 2026 10:20:59 +0200

On Wed, 24 Jun 2026 at 10:00, PG Bug reporting form
<[email-masked]> wrote:
...
> Inconsistent Error Messages for the Same SQL Query in PostgreSQL
...