Re: Options to rowwise persist result of stable/immutable function with RECORD result

On Wed, Mar 22, 2023 at 4:32 PM Eske Rahn <eske(at)septima(dot)dk> wrote:

> Hi,
>
> Thanks for the quick answer *:-D*
>
> That was a nice sideeffect of lateral.
>
> In the example, the calling code also gets simplified:
>
> WITH x AS (
> SELECT clock_timestamp() rowstart, *, clock_timestamp() rowend FROM (
> SELECT '1' inp UNION
> SELECT '2'
> ) y, LATERAL septima.foo(inp) g
> )
> SELECT * FROM x;
>
>
> That solved the issue at hand, in a much better way. Thanks
>
> Though I still fail to see *why* the other way should generally call the
> function for every column in the *result* record - if the function is
> STABLE or IMMUTABLE.
>

It gets rewritten to be effectively:

select func_call(...).col1, func_call(...).col2, func_call(...).col3

under the assumption that repeating the function call will be cheap and
side-effect free. It was never ideal but fixing that form of optimization
was harder than implementing LATERAL where the multi-column result has a
natural output in the form of a multi-column table. A normal function call
in the target list really means "return a single value" which is at odds
with writing .* after it.

David J.