Re: PostgreSQL vs SQL/XML Standards

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.

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.

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