Stange "duplicate key value violates unique constraint" after "delete" at ON UPDATE trigger

[Dmitry Koterov]

Hello.

PostgreSQL 8.4. I am trying to create a trigger which removes "old" rows
from the table just before inserting a new one. The table has an UNIQUE
INDEX on a column, that's why I need to remove an old item with the same
value of the column before inserting a new one.

If I work without transactions (in auto-commit mode), all seems to be
fine. But something strange is happened when I use transactions.

The following SQL represents the problem. How to avoid strange "duplicate
key value violates unique constraint" error (with minimum locking level)?..
And why this error happens at all?

-- Prepare the fixture.
create table a(i integer);
CREATE UNIQUE INDEX a_idx ON a USING btree (i);
CREATE FUNCTION a_tr() RETURNS trigger AS
$body$
BEGIN
DELETE FROM a WHERE i = NEW.i;
RETURN NEW;
END;
$body$
LANGUAGE 'plpgsql';
CREATE TRIGGER a_tr BEFORE INSERT ON a FOR EACH ROW EXECUTE PROCEDURE
a_tr();


-- Check if the trigger really works. No unique constraint errors are
thrown.
insert into a values(1);
insert into a values(1); --> ok


-- NOW IN CONNECTION (A):
begin;
insert into a values(1); --> do not commit!

-- THEN IN CONNECTION (B):
insert into a values(1); --> it hangs, because the
connection (A) is not committed - ok

-- NOW IN CONNECTION (A) AGAIN:
commit; --> ok

-- WE SEE AT CONNECTION (B) THE THE FOLLOWING IMMEDIATELY:
ERROR: duplicate key value violates unique constraint
"a_idx"

[Julian v. Bock]

Hi
DK> create table a(i integer);
DK> CREATE UNIQUE INDEX a_idx ON a USING btree (i);
DK> CREATE FUNCTION a_tr() RETURNS trigger AS
DK> $body$
DK> BEGIN
DK> DELETE FROM a WHERE i = NEW.i;
DK> RETURN NEW;
DK> END;
DK> $body$
DK> LANGUAGE 'plpgsql';
DK> CREATE TRIGGER a_tr BEFORE INSERT ON a FOR EACH ROW EXECUTE PROCEDURE
DK> a_tr();

The DELETE doesn't see the row the other transaction inserted and
doesn't delete anything (and doesn't block). This happens later when the
row is inserted and the index is updated.

You can try the insert and catch the unique violation in a loop (see
http://www.postgresql.org/docs/9.1/static/plpgsql-control-structures.html)
although that won't work with a BEFORE trigger.

Regards,
Julian
--
Julian v. Bock Projektleitung Software-Entwicklung
OpenIT GmbH Tel [phone]
In der Steele 33a-41 Fax [phone]
D-40599 Düsseldorf http://www.openit.de
________________________________________________________________
HRB 38815 Amtsgericht Düsseldorf USt-Id DE 812951861
Geschäftsführer: Oliver Haakert, Maurice Kemmann

[Dmitry Koterov]

I'm not sure the cause is that DELETE does not see the row.

Seems the following method solves the problem when 2 same-time transactions
are active:

CREATE FUNCTION a_tr() RETURNS trigger AS
$body$
DECLARE
tmp INTEGER;
BEGIN
-- Lock until the mathed entry (possibly phantom - i.e. not yet
committed
-- by another transaction) is released.
SELECT i INTO tmp FROM a WHERE i = NEW.i FOR UPDATE;

-- The lock is released here in one of two cases:
-- 1. Matched row was phantom, so tmp IS NULL now.
-- 2. Matched row was real and committed, so tmp holds its ID.
-- So we cannot use ID in tmp - it is not always returned. That's why
we have to
-- duplicate the selection predicate above...
DELETE FROM a WHERE i = NEW.i;

RETURN NEW;
END;
$body$
LANGUAGE 'plpgsql';

But this method still does not work if 3 or more transactions are active
(if I commit first and commit second, the third fails with "duplicate key"
error).

Are there any universal method which could be implemented purely in a
trigger?..

Post by Julian v. Bock
Hi
DK> create table a(i integer);
DK> CREATE UNIQUE INDEX a_idx ON a USING btree (i);
DK> CREATE FUNCTION a_tr() RETURNS trigger AS
DK> $body$
DK> BEGIN
DK> DELETE FROM a WHERE i = NEW.i;
DK> RETURN NEW;
DK> END;
DK> $body$
DK> LANGUAGE 'plpgsql';
DK> CREATE TRIGGER a_tr BEFORE INSERT ON a FOR EACH ROW EXECUTE PROCEDURE
DK> a_tr();
The DELETE doesn't see the row the other transaction inserted and
doesn't delete anything (and doesn't block). This happens later when the
row is inserted and the index is updated.
You can try the insert and catch the unique violation in a loop (see
http://www.postgresql.org/docs/9.1/static/plpgsql-control-structures.html)
although that won't work with a BEFORE trigger.
Regards,
Julian
--
Julian v. Bock Projektleitung Software-Entwicklung
OpenIT GmbH Tel [phone]
In der Steele 33a-41 Fax [phone]
D-40599 Düsseldorf http://www.openit.de
________________________________________________________________
HRB 38815 Amtsgericht Düsseldorf USt-Id DE 812951861
Geschäftsführer: Oliver Haakert, Maurice Kemmann
--
http://www.postgresql.org/mailpref/pgsql-general

[Dmitry Koterov]

I have had 2 hours of experiments and finally I suppose that there is no
way to satisfy this unique constraint index from within a trigger with
non-whole-table locking. So Julian seems to be right (unfortunately). Only

LOOP
BEGIN
INSERT ...;
EXIT;
EXCEPTION WHEN unique_violation THEN
DELETE FROM ... WHERE <unique constraint predicate>;
END;
END LOOP;

construction helps. There seems to be no way to implement the same using
triggers only.

Post by Dmitry Koterov
I'm not sure the cause is that DELETE does not see the row.
Seems the following method solves the problem when 2 same-time
CREATE FUNCTION a_tr() RETURNS trigger AS
$body$
DECLARE
tmp INTEGER;
BEGIN
-- Lock until the mathed entry (possibly phantom - i.e. not yet
committed
-- by another transaction) is released.
SELECT i INTO tmp FROM a WHERE i = NEW.i FOR UPDATE;
-- 1. Matched row was phantom, so tmp IS NULL now.
-- 2. Matched row was real and committed, so tmp holds its ID.
-- So we cannot use ID in tmp - it is not always returned. That's why
we have to
-- duplicate the selection predicate above...
DELETE FROM a WHERE i = NEW.i;
RETURN NEW;
END;
$body$
LANGUAGE 'plpgsql';
But this method still does not work if 3 or more transactions are active
(if I commit first and commit second, the third fails with "duplicate key"
error).
Are there any universal method which could be implemented purely in a
trigger?..

Post by Julian v. Bock
Hi
DK> create table a(i integer);
DK> CREATE UNIQUE INDEX a_idx ON a USING btree (i);
DK> CREATE FUNCTION a_tr() RETURNS trigger AS
DK> $body$
DK> BEGIN
DK> DELETE FROM a WHERE i = NEW.i;
DK> RETURN NEW;
DK> END;
DK> $body$
DK> LANGUAGE 'plpgsql';
DK> CREATE TRIGGER a_tr BEFORE INSERT ON a FOR EACH ROW EXECUTE PROCEDURE
DK> a_tr();
The DELETE doesn't see the row the other transaction inserted and
doesn't delete anything (and doesn't block). This happens later when the
row is inserted and the index is updated.
You can try the insert and catch the unique violation in a loop (see
http://www.postgresql.org/docs/9.1/static/plpgsql-control-structures.html
)
although that won't work with a BEFORE trigger.
Regards,
Julian
--
Julian v. Bock Projektleitung Software-Entwicklung
OpenIT GmbH Tel [phone]
In der Steele 33a-41 Fax [phone]
D-40599 Düsseldorf http://www.openit.de
________________________________________________________________
HRB 38815 Amtsgericht Düsseldorf USt-Id DE 812951861
Geschäftsführer: Oliver Haakert, Maurice Kemmann
--
http://www.postgresql.org/mailpref/pgsql-general