Re: BUG #16368: Incorrect function inlining in the presence of a window function

On Wednesday, April 15, 2020 11:32:54 A.M. PDT David G. Johnston wrote:
> During inlining the case expression becomes:
>
> CASE WHEN q.fmt IS NULL
> THEN '123,456'::bigint
> ELSE to_number('123,456', q.fmt)
> END;
>
> It doesn't "take" a branch - it turns variables into constants and, as
> written, some of those constants are invalid for the types they are
> being assigned to.
>
> > This only happens in the
> > presence of the "first_value" window call in the nested query.
>
> The ability to optimize, and how, depends on the whole query.
>
> I don't actually know whether this is a bug or just an expected
> downside to using inline-able functions and case statements to avoid
> malformed data parsing.
>
> Writing the function in pl/pgsql prevents the inlining and stabilizes
> the query.

IMO, an optimization that leads to wrong query results is unquestionably
a bug. And "use pl/pgsql" (which is much slower) is arguably not the
best answer here.

> inline-able functions and case statements to avoid
> malformed data parsing.

Consider any other case where an error is guarded by a "CASE WHEN", such
as division by zero. I think the use of "CASE WHEN" should disqualify
the function from being inlined, or, maybe, constant folding should be
disabled in the branches of "CASE WHEN" when inlining and when the
optimizer is unable to reason about the "CASE" condition.

Thoughts?

Elvis