Jim Nasby wrote:
> On 8/24/15 9:49 AM, Alexander Korotkov wrote:
> >2) Non-index access methods reuse pg_class.relam but don't reuse pg_am.
> >This violates relational theory because single column reference multiple
> >tables.
> >3) Non-index access methods reuse both pg_class.relam and pg_am. This
> >violates relational theory because we store different objects in the
> >same table.
> >
> >I'd say we already have precedent of #2. It's pg_depend which reference
> >objects of arbitrary types.
> >In the #3 we really shouldn't keep any specific to index am in pg_am.
In my reading of the thread, we have a consensus for doing #3, and that
one gets my vote in any case.
> In userspace, table inheritance handles this nicely. Stick a "type" field in
> the parent so you know what kind of entity each record is, along with all
> your common fields.
Yeah, this pattern is not hugely common but it's definitely used in some
places. In fact, I would think it is less of a violation of relational
theory than #2 -- because then relam is always a reference to pg_am,
instead of sometimes being a reference to some other catalog. What's
stored in pg_am is not pg_class' concern; and I think calling pg_am a
catalog for "access methods" (in a generic way, not only indexes) is
sound.
--
Álvaro Herrera http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services