Re: PostgreSQL vs SQL/XML Standards

ne 20. 1. 2019 23:13 odesílatel Chapman Flack <chap(at)anastigmatix(dot)net>
napsal:

> On 01/20/19 12:48, Pavel Stehule wrote:
> >>
> >> Accordingly, I think the paragraph beginning "Unlike regular PostgreSQL
> >> functions," is more likely to perplex readers than to enlighten them.
> >> What it says about column_expression does not seem to lead to any useful
> >> difference from the behavior if it were "just like regular PostgreSQL
> >> functions".
> >
> > regular Postgres' functions has evaluated all arguments before own
> > execution. I think so this note is related much more to expressions used
> as
> > defaults.
>
> Sure, but again, is there an example, or can one easily be constructed,
> that shows the default expressions working in such a way?
>
> I am not able to use a default expression to refer to an earlier
> column in the column list of the xmltable call.
>

probably you can see the effect if you use some volatile function ..
random(), nextval(),

I think so notice in documentation was not a motivation to use it. It was
explanation of implementation and warnings against side effect.

>
> I am able to use a default expression to refer to a column of an earlier
> FROM item in the enclosing SELECT. But such a query ends up having LATERAL
> form (whether or not the word LATERAL is used), and re-executes xmltable
> whenever the referenced column value changes. In that case, whether the
> default argument is evaluated at function entry or later doesn't seem
> to matter: the function is re-executed, so evaluating the new default
> at the time of entry is sufficient.
>

it has sense only for volatile functions. it was not often. On second hand
deferred evaluation shoul not be a problem, and more, it is evaluated only
when it is necessary, what is more sensible for me.

> So, I have still not been able to construct a query that requires the
> deferred evaluation behavior. But perhaps there is a way I haven't
> thought of.
>
> Regards,
> -Chap
>