Re: Assigning NULL to a record variable

2012/9/20 Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us>:
> "Kevin Grittner" <kgrittn(at)mail(dot)com> writes:
>> ... This did not work for cases where the AFTER DELETE trigger performed
>> an action which was not idempotent because, while return_value was
>> NULL enough to enter that last IF clause, it was not NULL enough to
>> prevent the DELETE attempt and fire subsequent triggers. The
>> assignment of NULL to a variable with a record type doesn't assign a
>> "simple" NULL, but a record with NULL in each element.
>
> I believe that this is forced by plpgsql's implementation. IIRC, a
> declared variable of a named composite type (not RECORD) is implemented
> as a "row" structure, meaning it actually consists of a separate plpgsql
> variable for each column. So there's no physical way for it to represent
> a "simple NULL" composite value.
>
> I've suggested in the past that we might want to go over to treating
> such variables more like RECORD, ie the representation is always a
> HeapTuple.

I had same idea when I worked on SQL/PSM - but there is significant
difference in performance (probably less in real tasks)

I'm not sure what the performance tradeoffs would be ---
> some things would get faster and others slower, probably, since field
> access would be more work but conversion to/from HeapTuple would get far
> cheaper.

when fields are fix length, then field's update is significantly
faster then when RECORD is used

>
>> - If we keep this behavior, should the triggering operation be
>> suppressed when (NOT return_value IS NOT NULL) instead of when
>> (return_value IS NOT DISTINCT FROM NULL)?
>
> Can't do that, because it would break the perfectly-legitimate case
> where the trigger is trying to process a row of all nulls.
>
> regards, tom lane
>
>
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