Re: SQL Property Graph Queries (SQL/PGQ)

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 ...

> Many people will agree that the rewriter is sort of weird and archaic at
> this point.  But I'm not aware of any plans or proposals to do anything
> about it.  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.  So
> for now, I'm working with what we have, but let's see where it goes.
> 
> (Note to self: Check that graph inside view inside graph inside view ...
> works.)
> 

AFAIK the "policy" regarding rewriter was that we don't want to use it
for user stuff (e.g. people using it for partitioning), but I'm not sure
about internal stuff.


regards

-- 
Tomas Vondra
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