Re: Removing useless DISTINCT clauses

On Thu, Mar 22, 2018 at 5:20 PM, David Rowley <david(dot)rowley(at)2ndquadrant(dot)com>
wrote:

> The problem is that in order to properly invalidate cached plans we
> must record the constraint OIDs which the plan depends on. We can do
> that for PK and UNIQUE constraints, unfortunately, we can't do it for
> NOT NULL constraints as, at the moment, these are just properties of
> pg_attribute. For this to be made to work we'd need to store those in
> pg_constraint too, which is more work that I'm going to do for this
> patch.
>

I was just wondering, since get_primary_key_attnos opens pg_constraint and
just skips attributes with other constraint types than primary, what would
be the reason we wouldn't just also open pg_attribute and check for the
non-nullness of the non-primary key unique attributes?
Couldn't we add a function which opens both pg_attribute and pg_constraint
to get non-null unique attributes?
I suppose that would have a performance impact.
And, I did notice the FIXME in the comment before check_functional_grouping
which seems to make the argument that there are other use cases for wanting
the non-nullness represented in pg_constraint.

* Currently we only check to see if the rel has a primary key that is a
* subset of the grouping_columns. We could also use plain unique
constraints
* if all their columns are known not null, but there's a problem: we need
* to be able to represent the not-null-ness as part of the constraints
added
* to *constraintDeps. FIXME whenever not-null constraints get represented
* in pg_constraint.
*/
--
Melanie Plageman