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Re: BUG #4997: Expression evaluation rules

From: Pavel Stehule <pavel(dot)stehule(at)gmail(dot)com>
To: Dmitry Samokhin <sdld(at)mail(dot)ru>
Cc: pgsql-bugs(at)postgresql(dot)org
Subject: Re: BUG #4997: Expression evaluation rules
Date: 2009-08-20 09:33:15
Message-ID: 162867790908200233p1167a00cu9d963be790d76d3@mail.gmail.com (view raw or flat)
Thread:
Lists: pgsql-bugs
2009/8/20 Dmitry Samokhin <sdld(at)mail(dot)ru>:
>
> The following bug has been logged online:
>
> Bug reference:      4997
> Logged by:          Dmitry Samokhin
> Email address:      sdld(at)mail(dot)ru
> PostgreSQL version: 8.3.7
> Operating system:   Windows 2003 Server
> Description:        Expression evaluation rules
> Details:
>
> As described in section "4.2.12. Expression Evaluation Rules" in the
> documentation, "the order of evaluation of subexpressions is not defined",
> and the proposed solution is to use the CASE statement. Consider the
> following query:
>
> SELECT CASE WHEN TRUE THEN TRUE ELSE 1 / 0 = 1 END;
>
> It returns 'TRUE' as expected, I don't see the 'division by zero' error, so
> the ELSE branch is not evaluated at all.
>
> Then, consider this test case:
>
> -- Start of DDL script
>
> CREATE OR REPLACE FUNCTION trg_mytable_after()
>  RETURNS trigger AS
> $BODY$
> BEGIN
>  IF TG_OP = 'UPDATE' OR TG_OP = 'DELETE' THEN
>    IF (CASE WHEN TG_OP = 'DELETE' THEN TRUE ELSE OLD.a <> NEW.a END) THEN
>      RAISE NOTICE 'OK!';
>    END IF;
>  END IF;
>
>  RETURN NULL;
> END;
> $BODY$
>  LANGUAGE 'plpgsql' VOLATILE
>  COST 100;
>
> CREATE TABLE mytable
> (
>  a integer NOT NULL,
>  CONSTRAINT pk_mytable PRIMARY KEY (a)
> )
> WITH (
>  OIDS=FALSE
> );
>
> CREATE TRIGGER trg_mytable_after
>  AFTER INSERT OR UPDATE OR DELETE
>  ON mytable
>  FOR EACH ROW
>  EXECUTE PROCEDURE trg_mytable_after();
>
> -- End of DDL script
>
> INSERT INTO mytable (a) VALUES (1);
> DELETE FROM mytable WHERE a = 1;
>
> On DELETE statement, the error occurs:
>
> ERROR record "new" is not assigned yet
> DETAIL The tuple structure of a not-yet-assigned record is indeterminate
>
> So it seems although (TG_OP = 'DELETE') is TRUE, the ELSE branch of the CASE
> statement (OLD.a <> NEW.a) in the trigger procedure is still evaluated.
>
> --
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>

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Next:From: Pavel StehuleDate: 2009-08-20 09:42:06
Subject: Re: BUG #4997: Expression evaluation rules
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Subject: BUG #4997: Expression evaluation rules

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