Re: Postgres 11: Table Partitioning and Primary Keys

On Wed, Jul 10, 2019 at 7:53 AM Alvaro Herrera <alvherre(at)2ndquadrant(dot)com> wrote:
> On 2019-Jul-09, Amit Langote wrote:
> > "While defining a primary key and unique constraints on partitioned
> > tables is supported, the set of columns being constrained must include
> > all of the partition key columns. This limitation exists because
> > <productname>PostgreSQL</productname> can ensure uniqueness only
> > across a given partition."
>
> I feel that PKs are mostly a special case of UNIQUE keys, so I tend to
> mention UNIQUE as the central element and let PKs fall out from that.
> That's a mild personal preference only though. Anyway, based on your
> proposed wording, I wrote this:
>
> <listitem>
> <para>
> Unique constraints on partitioned tables (as well as primary keys)
> must constrain all the partition key columns. This limitation exists
> because <productname>PostgreSQL</productname> can only enforce
> uniqueness in each partition individually.
> </para>
> </listitem>
>
> I'm not really sure about the "must constrain" verbiage. Is that really
> comprehensible?

Looks good after replacing "must constraint" by "must include" as
suggested by Tom.

> Also, I chose to place it just above the existing para
> that mentions FK limitations

This placement of the new text sounds good.

> In CREATE TABLE, we already have this:
> <para>
> When establishing a unique constraint for a multi-level partition
> hierarchy, all the columns in the partition key of the target
> partitioned table, as well as those of all its descendant partitioned
> tables, must be included in the constraint definition.
> </para>
>
> which may not be the pinnacle of clarity, but took some time to craft
> and I think is correct. Also it doesn't mention primary keys
> explicitly; maybe we should patch it by adding "(as well as a primary
> key)" right after "a unique constraint". Thoughts?

Works for me.

Thanks,
Amit