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From: Chapman Flack <chap@anastigmatix.net>
To: Pavel Stehule <pavel.stehule@gmail.com>
Cc: PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>
Subject: Re: PostgreSQL vs SQL/XML Standards
Date: Sun, 20 Jan 2019 17:13:01 -0500
Message-ID: <5C44F26D.5010703@anastigmatix.net> (raw)
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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




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