From: Mladen Gogala <gogala.mladen@gmail.com>
To: pgsql-performance@lists.postgresql.org
Subject: Re: Simple task with partitioning which I can't realize
Date: Wed, 2 Mar 2022 09:04:20 -0500
Message-ID: <c5f998a9-2de6-9bba-9b83-2f838140a718@gmail.com> (raw)
In-Reply-To: <CAKFQuwZbJObQmVcCOYNiYS_PHu2n39xZzoFY_4KZLSZ6MWuPsQ@mail.gmail.com>
References: <011d01d82d82$42d20c50$c87624f0$@mail.ru>
<CAKFQuwZbJObQmVcCOYNiYS_PHu2n39xZzoFY_4KZLSZ6MWuPsQ@mail.gmail.com>
On 3/1/22 10:54, David G. Johnston wrote:
> On Tue, Mar 1, 2022 at 8:37 AM Andrew Zakharov <Andrew898@mail.ru> wrote:
>
> create table region_hierarchy(
>
> gid uuid not null default uuid_generate_v1mc(),
>
> parent_gid uuid null,
>
> region_code int2,
>
> I’ve carefully looked thru docs/faqs/google/communities and found
> out that I must include “gid” field into partition key because a
> primary key field.
>
>
> Yes, you are coming up against the following limitation:
>
> "Unique constraints (and hence primary keys) on partitioned tables
> must include all the partition key columns. This limitation exists
> because the individual indexes making up the constraint can only
> directly enforce uniqueness within their own partitions; therefore,
> the partition structure itself must guarantee that there are not
> duplicates in different partitions."
>
> https://www.postgresql.org/docs/current/ddl-partitioning.html#DDL-PARTITIONING-DECLARATIVE
>
> That limitation is independent of partitioning; i.e., the legacy
> inheritance option doesn't bypass it.
>
> Thus, your true "key" is composite: (region, identifier). Thus you
> need to add a "parent_region_code" column as well, redefine the PK as
> (region_code, gid), and the REFERENCES clause to link the two paired
> fields.
>
> You can decide whether that is sufficient or if you want some added
> comfort in ensuring that a gid cannot appear in multiple regions by
> creating a single non-partitioned table containing all gid values and
> add a unique constraint there.
>
> Or maybe allow for duplicates across region codes and save space by
> using a smaller data type (int or bigint - while renaming the column
> to "rid" or some such) - combined with having the non-partitioned
> reference table being defined as (region_code, rid, gid).
>
> David J.
>
Hi David,
Are there any concrete plans to address that particular limitation? That
limitation can be re-stated as "PostgreSQL doesn't support global
indexes on the partitioned tables" and I've have also run into it. My
way around it was not to use partitioning but to use much larger machine
with the NVME disks, which can handle the necesary I/O. Are there any
plans to allow global indexes? I am aware that this is not a small
change but is the only real advantage that Oracle holds over PostgreSQL.
Regards
--
Mladen Gogala
Database Consultant
Tel: (347) 321-1217
https://dbwhisperer.wordpress.com
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Subject: Re: Simple task with partitioning which I can't realize
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