Re: plans for PostgreSQL 12

On 04/06/18 09:12, Pavel Stehule wrote:
> 2018-06-04 8:35 GMT+02:00 Simon Riggs <simon@2ndquadrant.com>:
>>
>> Sounds good. I think this would need to be restricted by operator and
>> datatype, since in general you won't know if the datatype functions
>> need a snapshot or not. Immutable functions for the operators ought to
>> do it, but I think that might not be enough.
> 
> It requires introduction of new "safe" functions (& operators). Immutable
> functions are not enough safe.
> 
> CREATE OR REPLACE FUNCTION public.fx()
>   RETURNS integer
>   LANGUAGE plpgsql
>   IMMUTABLE
> AS $function$
> BEGIN
>    RETURN (SELECT count(*) FROM pg_class);
> END;
> $function$
> 
> postgres=# SELECT fx();
> ┌─────┐
> │ fx  │
> ╞═════╡
> │ 343 │
> └─────┘
> (1 row)

That function is incorrectly marked as IMMUTABLE. In that situation, 
it's enough that we throw a sane error like "ERROR: no snapshot available".

- Heikki