Re: [SQL] plpgsql doesn't coerce boolean expressions to

Tom Lane wrote:

> Jan Wieck <JanWieck(at)Yahoo(dot)com> writes:
>> Tom Lane wrote:
>>>> 4. Use the parser's coerce_to_boolean procedure, so that nonbooleans
>>>> will be accepted in exactly the same cases where they'd be accepted
>>>> in a boolean-requiring SQL construct (such as CASE). (By default,
>>>> none are, so this isn't really different from #2. But people could
>>>> create casts to boolean to override this behavior in a controlled
>>>> fashion.)
>
>> Agreed - #4.
>
> My first attempt at doing this failed to pass the regression tests,
> because it wasn't prepared for this:
>
> if count(*) = 0 from Room where roomno = new.roomno then
> raise exception ''Room % does not exist'', new.roomno;
> end if;
>
> Is this really intended to be a feature? It manages to work because
> plpgsql simply sticks "SELECT " in front of whatever appears between
> IF and THEN, and passes the result to the main SQL engine. But it sure
> surprised the heck out of me. The documentation gives no hint that
> you're allowed to write anything but a straight boolean expression in IF.
> Does Oracle allow that sort of thing?

I have to admit it was less an intention than more a side effect of the
actual implementation. It was so easy to simply stick "SELECT " in front
of "everything between IF and THEN" and expect the result to be a boolean.

In the same way you can do

varname := count(*) from Room where roomno = new.roomno;

which is straight forward because it's simply sticking "SELECT " in
front of "everything between := and ;". Well, this does a bit more in
that it tries the typinput(typoutput(result)) casting hack ... I know
that you don't like that one.

>
> I would be inclined to think that a more reasonable expression of the
> intent would be
>
> if (select count(*) from Room where roomno = new.roomno) = 0 then
>
> Certainly we'd have a big problem supporting the existing coding if we
> ever reimplement plpgsql with more awareness of what expressions are.

Without parsing much, much more, and finally parsing basically the whole
SQL grammar in the PL/pgSQL parser, I don't see how you can do that.

Jan

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