Re: StrategyAM for IndexAMs

On Fri, 22 Jul 2022 at 10:23, Matthias van de Meent
<boekewurm+postgres(at)gmail(dot)com> wrote:
>
> On Tue, 19 Jul 2022 at 18:56, Simon Riggs <simon(dot)riggs(at)enterprisedb(dot)com> wrote:
> >
> > I'm preparing the way for a later patch that would allow unique hash
> > indexes to be primary keys. There are various parts to the problem. I
> > was surprised at how many times we hardcode BTREE_AM_OID and
> > associated BT Strategy Numbers in many parts of the code, so have been
> > looking for ways to reconcile how Hash and Btree strategies work and
> > potentially remove hardcoding. There are various comments that say we
> > need a way to be able to define which Strategy Numbers are used by
> > indexams.
> >
> > I came up with a rather simple way: the indexam just says "I'm like a
> > btree", which allows you to avoid adding hundreds of operator classes
> > for the new index, since that is cumbersome and insecure.
>
> I'm fairly certain that you can't (and don't want to) make a hash
> index look like a btree index, considering that of the btree
> operations only equality checks make sense in the hash context, and
> that you can't do ordered retrieval (incl. no min/max), which are
> major features of btree.

"like a $INDEX_TYPE" is wrong. What I really mean is "use the operator
strategy numbering same as $INDEX_TYPE".

> With that in mind, could you tell whether this patch is related to the
> effort of hash-based unique primary keys (apart from inspiration
> during development), and if so, how?

There are lots of places that are hardcoded BTREE_AM_OID, with a mix
of purposes. It's hard to tackle one without getting drawn in to fix
the others.

> > Specifically, we add a "strategyam" field to the IndexAmRoutine that
> > allows an indexam to declare whether it is like a btree, like a hash
> > index or another am. This then allows us to KEEP the hardcoded
> > BTREE_AM_OID tests, but point them at the strategyam rather than the
> > relam, which can be cached in various places as needed. No catalog
> > changes needed.
> >
> > I've coded this up and it works fine.
> >
> > The attached patch is still incomplete because we use this in a few
> > places and they all need to be checked. So before I do that, it seems
> > sensible to agree the approach.
> >
> > (Obviously, there are hundreds of places where BTEqualStrategyNumber
> > is hardcoded, and this doesn't change that at all, in case that wasn't
> > clear).
> >
> > Comments welcome on this still WIP patch.
>
> I think this is a great step in the right direction, fixing one of the
> issues with core index AMs, issues I also complained about earlier
> [0].
>
> [0] https://www.postgresql.org/message-id/CAEze2Wg8QhpOnHoqPNB-AaexGX4Zaij%3D4TT0kaMhF_6T5FXxmQ%40mail.gmail.com

Guess we're thinking along similar lines. I was unaware of your recent
post; these days I don't read Hackers apart from what I'm working on.

--
Simon Riggs http://www.EnterpriseDB.com/