Re: SQL Property Graph Queries (SQL/PGQ)

Hi Ashutosh,

Thanks for the patch and the analysis.

In my patch I have added a test for forward reference, but didn't
> implement the support for the same. I am not sure whether we want to
> support forward references right now, for the reasons mentioned below
>
> a. Other products allow cross referencing, but it seems it's optional
> in SQL standard. Conformance rule 11 of subclause 10.6 seems to make
> the feature optional. If that's the case, we need to throw an error in
> case of cross reference. Peter, what do you think?
>

I looked into this. Conformance Rule 11 says:

"Without Feature G041, 'Non-local element pattern predicates',
in conforming SQL language, the <element pattern where clause>
of an <element pattern> EP shall only reference the
<element variable> declared in EP."

This makes *all* cross-variable references in element WHERE clauses
optional under G041 -- it does not distinguish between backward and
forward references. So if we support G041 (which we effectively do,
since backward cross-references already work after the
list_make1(pe) -> graph_path fix), then SR 18 applies in full:

"The scope of an <element variable> that is declared by an
<element pattern> EP is the innermost <graph table> containing EP."

The current state -- backward yes, forward no -- is a
half-implementation that does not correspond to either option.

We should either:

(1) Support G041 fully -- both directions. Junwang's patch handles
this cleanly with minimal code change, or
(2) Not support G041 -- reject all non-local element pattern
references, throwing an error for both directions.

What do you think?

Regards,
Henson