Re: IMMUTABLE and PARALLEL SAFE function markings

On 27/11/2018 01:40, Tom Lane wrote:
> Vik Fearing <vik.fearing@2ndquadrant.com> writes:
>> On 27/11/2018 01:13, Stephen Frost wrote:
>>> Parallel safe functions should be marked as such.  Immutable functions
>>> should be marked as such.  We should not assume that one implies the
>>> other, nor should we operate as if they do.
> 
>> Yes we should!  Unless you can produce a case where an immutable
>> function is not parallel safe.
> 
> As far as that goes, I agree with the idea of adding an oprsanity test
> that shows any built-in functions that are immutable but not parallel
> safe, on the grounds Stephen mentioned: it's probably a mistake, and
> if it isn't, we can add that function to the expected output.
> 
> I'm way less inclined to buy into the idea that it MUST be wrong, though.
> Immutability is a promise about result stability and lack of side effects,
> but it is not a promise about implementation details.  There could be an
> implementation reason not to run something in a parallel worker.  Off the
> top of my head, a possible example is "it's written in plfoo which hasn't
> yet been made to work correctly in parallel workers".

Now, see, that is an actual argument for making a difference.  The other
arguments in this thread were not, so say I.
-- 
Vik Fearing                                          +33 6 46 75 15 36
http://2ndQuadrant.fr     PostgreSQL : Expertise, Formation et Support