Re: BUG #6128: A boolean variable doesn't evaluate properly in an IF conditional...

On Thu, Jul 21, 2011 at 2:48 PM, <danielb(at)megasystems(dot)us> wrote:
> I wrote some code in a PLPGSQL function to generate a printout string of
> some information. Basically, I create a string of printed characters, which
> my calling program will then print out...
>
> I wrote an IF conditional to generate different printing based on what input
> data I got...
>
> I declared the variable "put_the_in_city_online boolean := false" in my
> declaration section. I then had the following IF conditional inside a FOR
> LOOP structure with a record variable called "rec"...
>
> -- If the applicant has electric services,
> -- then we display a line indicating such...
> IF (rec.has_electric = 'Y') THEN
>  IF (put_the_in_city_online = FALSE) THEN
>    application_page_01 := application_page_01 ||
>      '  SERVICES: ELECTRIC   ' ||
>      rec.electric_rate_category || '              '
>      || rec.in_or_out_of_city || end_of_line;
>
>          put_the_in_city_online := true;
>  ELSE
>    application_page_01 := application_page_01 ||
>      '            ELECTRIC   ' ||
>      rec.electric_rate_category || end_of_line;
>  END IF;
> END IF;
>
> The bug occurs in the execution of these statements. I enter inside the
> first IF statement no problem, but the 2nd IF statement executes the
> opposite of what it should. The ELSE code is executing when the IF code
> should be executing!
>
> For some reason, POSTGRESQL does not evaluate ((put_the_in_city_online =
> FALSE) = true) correctly. This statement is true, but corresponding
> statements are not executed; the statements for ((put_the_in_city_online =
> FALSE) = false) end up being executed!
>
> I don't see any flaw in my logic. Postgresql doesn't seem to work correctly
> based on logic!

I can't help wondering if you are somehow getting bitten by the
strange semantics of NULL comparisons. Note that:

IF (something_that_is_null) THEN
...
END IF;

...will not trigger the if, and:

IF (something_that_is_null = false) THEN
...
END IF;

won't either.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company