Re: Documentation correction suggestion for primary key hashing on partitioned tables

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:
>
> > To create a unique or primary key constraint on a partitioned table, the
> 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; therefore,
> the partition structure itself must guarantee that there are not duplicates
> in different partitions.
>
> 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.

From the above:

  To create a unique or primary key constraint on a partitioned table,
  [...] the constraint's columns must include all of the partition key columns.

You seem to read it the other way around.  Your example confirms what the
documentation says.

Yours,
Laurenz Albe