Re: SQL Property Graph Queries (SQL/PGQ)

On Fri, Feb 23, 2024 at 11:08 PM Tomas Vondra
<tomas.vondra@enterprisedb.com> wrote:
>
> On 2/23/24 17:15, Peter Eisentraut wrote:
> > On 16.02.24 20:23, Andres Freund wrote:
> >> One aspect that I m concerned with structurally is that the
> >> transformation,
> >> from property graph queries to something postgres understands, is done
> >> via the
> >> rewrite system. I doubt that that is a good idea. For one it bars the
> >> planner
> >> from making plans that benefit from the graph query formulation. But more
> >> importantly, we IMO should reduce usage of the rewrite system, not
> >> increase
> >> it.
> >
> > PGQ is meant to be implemented like that, like views expanding to joins
> > and unions.  This is what I have gathered during the specification
> > process, and from other implementations, and from academics.  There are
> > certainly other ways to combine relational and graph database stuff,
> > like with native graph storage and specialized execution support, but
> > this is not that, and to some extent PGQ was created to supplant those
> > other approaches.
> >
>
> I understand PGQ was meant to be implemented as a bit of a "syntactic
> sugar" on top of relations, instead of inventing some completely new
> ways to store/query graph data.
>
> But does that really mean it needs to be translated to relations this
> early / in rewriter? I haven't thought about it very deeply, but won't
> that discard useful information about semantics of the query, which
> might be useful when planning/executing the query?
>
> I've somehow imagined we'd be able to invent some new index types, or
> utilize some other type of auxiliary structure, maybe some special
> executor node, but it seems harder without this extra info ...

I am yet to look at the implementation but ...
1. If there are optimizations that improve performance of some path
patterns, they are likely to improve the performance of joins used to
implement those. In such cases, loosing some information might be ok.
2. Explicit graph annotatiion might help to automate some things like
creating indexes automatically on columns that appear in specific
patterns OR create extended statistics automatically on the columns
participating in specific patterns. OR interpreting statistics/costing
in differently than normal query execution. Those kind of things will
require retaining annotations in views, planner/execution trees etc.
3. There are some things like aggregates/operations on paths which
might require stuff like new execution nodes. But I am not sure we
have reached that stage yet.

There might be things we may not see right now in the standard e.g.
indexes on graph properties. For those mapping the graph objects unto
database objects might prove useful. That goes back to Peter's comment
--- quote
As long as the view expansion takes place there, it makes
sense to align with that.  For example, all the view security stuff
(privileges, security barriers, etc.) will eventually need to be
considered, and it would make sense to do that in a consistent way.
--- unquote

--
Best Wishes,
Ashutosh Bapat