Re: PG11 Hash partitioning and null values in the partition key

On Fri, Jul 13, 2018, 7:35 PM Daniel Westermann <
daniel(dot)westermann(at)dbi-services(dot)com> wrote:

> Hi,
>
> given this setup:
>
> create table part2 ( a int, list varchar(10) ) partition by hash (a);
> create table part2_1 partition of part2 FOR VALUES WITH (MODULUS 3,
> REMAINDER 0);
> create table part2_2 partition of part2 FOR VALUES WITH (MODULUS 3,
> REMAINDER 1);
> create table part2_3 partition of part2 FOR VALUES WITH (MODULUS 3,
> REMAINDER 2);
>
> insert into part2 (a,list) values (1,'aaa');
> insert into part2 (a,list) values (2,'bbb');
> insert into part2 (a,list) values (3,'ccc');
>
> ... it is possible to insert rows like this which will always go to the
> first partition:
>
> insert into part2 (a,list) values (null,'ddd');
> insert into part2 (a,list) values (null,'eee');
> select * from part2_1;
> a | list
> ---+------
> 2 | bbb
> | ddd
> | eee
> (3 rows)
>
> I suppose this is intended but I could not find anything in the
> documentation about that. Can someone please clarify the logic behind that?
>

The calculated hash value for the null value will be zero, therefore, it
will fall to the partition having remainder zero.

​Regards,
Amul​