Horton

Re: PostgreSQL vs SQL/XML Standards

Chapman Flack <chap@anastigmatix.net>

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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>
Date: 2019-01-20T22:13:01Z
Lists: pgsql-hackers 
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

Commits

  1. Improve documentation about our XML functionality

  2. Improve documentation about our XML functionality.

  3. Add volatile qualifier missed in commit 2e616dee9.

  4. Fix crash with old libxml2

  5. Fix minor deficiencies in XMLTABLE, xpath(), xmlexists()

  6. Fix the BY {REF,VALUE} clause of XMLEXISTS/XMLTABLE

  7. doc: Update README.links

  8. XPath fixes: