RE: Potentially undocumented behaviour change in Postgres 11 concerning OLD record in an after insert trigger

Hello

From: Kristjan Tammekivi <kristjantammekivi(at)gmail(dot)com>
Sent: Freitag, 4. Januar 2019 11:46
To: pgsql-general(at)postgresql(dot)org
Subject: Potentially undocumented behaviour change in Postgres 11 concerning OLD record in an after insert trigger

Hi,

I've noticed a change in the behaviour in triggers / hstores in Postgres 11.1 when compared to Postgres 10.5.

The following won't work on Postgres 10.5 but in Postgres 11.1 it works just fine:

CREATE EXTENSION hstore;

CREATE TABLE _tmp_test1 (id serial PRIMARY KEY, val INTEGER);
CREATE TABLE _tmp_test1_changes (id INTEGER, changes HSTORE);

CREATE FUNCTION test1_trigger ()
RETURNS TRIGGER
LANGUAGE plpgsql
AS
$BODY$
BEGIN
INSERT INTO _tmp_test1_changes (id, changes) VALUES (NEW.id, hstore(OLD) - hstore(NEW));
RETURN NEW;
END
$BODY$;

CREATE TRIGGER table_update AFTER INSERT OR UPDATE ON _tmp_test1
FOR EACH ROW EXECUTE PROCEDURE test1_trigger();

INSERT INTO _tmp_test1 (val) VALUES (5);

ERROR: record "old" is not assigned yet

DETAIL: The tuple structure of a not-yet-assigned record is indeterminate.

CONTEXT: SQL statement "INSERT INTO _tmp_test1_changes (id, changes) VALUES (NEW.id, hstore(OLD) - hstore(NEW))"

PL/pgSQL function test1_trigger() line 3 at SQL statement

I couldn't find anything about this in the release notes (https://www.postgresql.org/docs/11/release-11.html) but maybe I just didn't know what to look for.

I doubt that this works on any PG version for INSERT.

According to the documentation:

https://www.postgresql.org/docs/10/plpgsql-trigger.html and https://www.postgresql.org/docs/11/plpgsql-trigger.html

OLD: Data type RECORD; variable holding the old database row for UPDATE/DELETE operations in row-level triggers. This variable is unassigned in statement-level triggers and for INSERT operations.

Regards

Charles