From: | Thomas Munro <thomas(dot)munro(at)enterprisedb(dot)com> |
---|---|
To: | Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> |
Cc: | Ruben Buchatskiy <ruben(at)ispras(dot)ru>, Pg Hackers <pgsql-hackers(at)postgresql(dot)org>, Amit Kapila <amit(dot)kapila16(at)gmail(dot)com> |
Subject: | Re: Non-deterministic behavior with floating point in parallel mode |
Date: | 2017-02-03 08:05:47 |
Message-ID: | CAEepm=2n7onP5aeypEYxAxgo0FX4eLRbALajzibkF8JhBKiZEw@mail.gmail.com |
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On Fri, Feb 3, 2017 at 3:49 AM, Tom Lane <tgl(at)sss(dot)pgh(dot)pa(dot)us> wrote:
> Ruben Buchatskiy <ruben(at)ispras(dot)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
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