Re: Multiple evaluation of single reference to function with out parameters

2017-09-23 4:49 GMT+02:00 Joel Hoffman <joel(dot)hoffman(at)gmail(dot)com>:

> Thanks for the response. Here's the documentation reference:
>
> https://www.postgresql.org/docs/9.6/static/rowtypes.html#ROWTYPES-USAGE
>
> Not a bug, but I do think it's very surprising behavior.
>

yes, but the fix is very simple - you just need only one level of nested
query more - or you call function from FROM clause.

The reason of this behave is given by implementation, that is very simple.
Nobody had too issues with it and nobody sent a patch to change it.

Regards

Pavel

> Joel
>
>
> On Fri, Sep 22, 2017 at 5:22 PM, David G. Johnston <
> david(dot)g(dot)johnston(at)gmail(dot)com> wrote:
>
>> On Friday, September 22, 2017, Joel Hoffman <joel(dot)hoffman(at)gmail(dot)com>
>> wrote:
>>
>>> If I create a function with more than one out parameter, and then refer
>>> to it inside parentheses as a record, e.g. select (function()).*, the
>>> function appears to be evaluated multiple times, once for every column
>>> returned. This seems to be true regardless of whether it's defined as
>>> volatile or immutable.
>>>
>>> ...
>>
>>> As far as I can tell, this behavior has been the same since at least
>>> version 8.2 and up through 10 beta 4, but I can't find any references to it
>>> and it seems very surprising. It could certainly cause unexpected results
>>> if the function has side effects. Is this a bug?
>>>
>>>
>> It is not a bug or likely to be fixed. LATERAL makes the need for
>> function invocation in the manner you describe nearly unnecessary and you
>> can use OFFSET 0 in other cases to put the function call in a subquery and
>> place the (composite).* expressionin the upper-level.
>>
>> There is a cautionary note somewhere in docs about this. It is a parser
>> byproduct. The star gets expanded at parse time to individual and
>> independent column names. What happens is exactly what you'd expect if you
>> tried to write the query without resorting to using ".*"
>>
>> David J.
>>
>
>