Re: [HACKERS] Non-deterministic behavior with floating point inparallel mode

On Fri, Feb 3, 2017 at 3:49 AM, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> Ruben Buchatskiy <ruben@ispras.ru> writes:
>> We have found that in parallel mode result of queries is non-deterministic
>> when the types of the attributes in table are double precision
>> (floating-point).
>
> Yeah ...
>
>> That is because floating-point addition is not necessarily associative.
>
> Right, exactly.
>
>> Is this desirable behavior?
>
> It's not especially the fault of parallelism.  Any change in the order in
> which the SUM visits the rows could cause a similar change in the results.
> IOW, you are being overoptimistic about how deterministic this result
> is any of the time.

For example, I just did the following while also running the same
query in another session to provoke synchronize_seqscans (in a
REPEATABLE READ transaction for added absurdity):

tpch=# set max_parallel_workers_per_gather to 0;
SET

tpch=# select sum(l_extendedprice::double precision) from lineitem;      sum
------------------229577310901.211
(1 row)

tpch=# select sum(l_extendedprice::double precision) from lineitem;      sum
------------------229577310901.198
(1 row)

-- 
Thomas Munro
http://www.enterprisedb.com