Message-ID: <b326de4ee8fb39593287c7f88c9fa9034e5b2feb.camel@cybertec.at>
Subject: Re: Documentation correction suggestion for primary key hashing on
 partitioned tables
From: Laurenz Albe <laurenz.albe@cybertec.at>
To: mark@markandruth.co.uk, pgsql-docs@lists.postgresql.org
Date: Thu, 24 Apr 2025 22:04:57 +0200
In-Reply-To: <174548810572.676.2745231019536109132@wrigleys.postgresql.org>
References: <174548810572.676.2745231019536109132@wrigleys.postgresql.org>
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On Thu, 2025-04-24 at 09:48 +0000, PG Doc comments form wrote:
> https://www.postgresql.org/docs/current/ddl-partitioning.html / 5.12.2.3.
> Limitations says:
>=20
> > To create a unique or primary key constraint on a partitioned table, th=
e
> partition keys must not include any expressions or function calls and the
> constraint's columns must include all of the partition key columns. This
> limitation exists because the individual indexes making up the constraint
> can only directly enforce uniqueness within their own partitions; therefo=
re,
> the partition structure itself must guarantee that there are not duplicat=
es
> in different partitions.
>=20
> This seems to imply that if I have a table of primary key (a, b, c) in
> b-tree index, then a partition-by-hash method would require this full set=
 of
> columns to be used in order for the constraints to be enforcable. However
> logically this should not be the case - only a hash of a prefix of the
> primary key - eg (a) or (a, b) should be needed to target the same table
> each time and hence get the constraint enforced. Testing this in postgres=
 16
> seems to show that it does work like this.