Re: Multiple evaluation of single reference to function with out parameters

On Friday, September 22, 2017, Joel Hoffman <joel(dot)hoffman(at)gmail(dot)com> wrote:

> If I create a function with more than one out parameter, and then refer to
> it inside parentheses as a record, e.g. select (function()).*, the function
> appears to be evaluated multiple times, once for every column returned.
> This seems to be true regardless of whether it's defined as volatile or
> immutable.
>
> ...

> As far as I can tell, this behavior has been the same since at least
> version 8.2 and up through 10 beta 4, but I can't find any references to it
> and it seems very surprising. It could certainly cause unexpected results
> if the function has side effects. Is this a bug?
>
>
It is not a bug or likely to be fixed. LATERAL makes the need for function
invocation in the manner you describe nearly unnecessary and you can use
OFFSET 0 in other cases to put the function call in a subquery and place
the (composite).* expressionin the upper-level.

There is a cautionary note somewhere in docs about this. It is a parser
byproduct. The star gets expanded at parse time to individual and
independent column names. What happens is exactly what you'd expect if you
tried to write the query without resorting to using ".*"

David J.