Re: HASH partitioning not working properly

On Fri, Jun 19, 2020 at 3:09 PM Amul Sul <sulamul(at)gmail(dot)com> wrote:

> On Fri, Jun 19, 2020 at 1:28 PM Srinivasa T N <seenutn(at)gmail(dot)com> wrote:
> >
> > On Fri, Jun 19, 2020 at 12:34 PM Laurenz Albe <laurenz(dot)albe(at)cybertec(dot)at>
> wrote:
> >>
> >> On Fri, 2020-06-19 at 12:12 +0530, Srinivasa T N wrote:
> >> > On Fri, Jun 19, 2020 at 11:44 AM David Rowley <dgrowleyml(at)gmail(dot)com>
> wrote:
> >> > > On Fri, 19 Jun 2020 at 17:42, Srinivasa T N <seenutn(at)gmail(dot)com>
> wrote:
> >> > > > After seeing the below, I feel partitioning is not working
> properly or it maybe case that my understanding is wrong. Can somebody
> explain me what is happening?
> >> > >
> >> > > It's your understanding that's not correct. The value of is passed
> >> > > through a hash function and the partition is selected based
> partition
> >> > > matching the remainder value after dividing the return value of the
> >> > > hash function by the largest modulus of any partition.
> >> > >
> >> > > That might surprise you, but how would you select which partition a
> >> > > varchar value should go into if you didn't use a hash function.
> >> > >
> >> > > David
> >> >
> >> > How can I see the output of hash function that is used internally?
> >>
> >> In the case of "integer", the hash function is "pg_catalog"."hashint4".
> >>
> >> Yours,
> >> Laurenz Albe
> >> --
> >> Cybertec | https://www.cybertec-postgresql.com
> >>
> > I guess output formatting is wrong, any help?
> >
> > postgres=# select pg_catalog.hashint4(7);
> > hashint4
> > ------------
> > -978793473
> > (1 row)
> >
> Instead of direct hash function, the easiest way to use
> satisfies_hash_partition() what is used in defining hash
> partitioning constraint.
>
> You can see the partition constraint by description partition table i.e.
> use \d+ busbar_version5.
>
> Regards,
> Amul
>

Sorry, I did not get you.

My current \d+ is

postgres=# \d+ busbar_version6;
Table "test.busbar_version6"
Column | Type | Collation | Nullable | Default | Storage | Stats
target |
Description
----------+---------+-----------+----------+---------+---------+--------------+-
------------
objectid | integer | | | | plain |
|
ver_id | integer | | | | plain |
|
Partition of: busbar_version FOR VALUES WITH (modulus 10, remainder 6)
Partition constraint: satisfies_hash_partition('16397'::oid, 10, 6, ver_id)
Access method: heap

Regards,
Seenu.