[SQL/PGQ] Custom executor node for VLE

Hello everyone,

After following the discussion from afar, I am interested in implementing the
custom executor node for VLE.

From what I understand, graph queries are rewritten into JOIN queries in order
to use the complete machinery available for optimization.
However, only single hop are currently allowed. While more complex queries are
parsed, an error is raised by the executor if it encounter this situation.
VLE are not allowed at all, and we are missing the capabilities to support
TRAIL/SIMPLE/ACYCLIC syntax, as well as SHORTEST and other selective path
search prefixes.

My plan was to keep the query rewriting for fixed-depth, and switch to the
custom executor in case of VLE, with VLE following the strategy from Henson
Choi ([1]) (with the correct syntax for the queries).

However, it seems a rather complex thing to keep with query rewriting, so I was
wondering if it would not be better (or at least simpler in the long run) to
create a full GraphNode that would handle graph traversal (with recursion),
and do away with the query rewrite.
This node would only do the minimum possible (like respecting edge label) and
simply send the possible paths to the rest of the planner, in order to let it
decide what to do with WHERE clauses.
This has the benefits of not isolating the graph queries from the rest of the
planner.

What are your thoughts ?

[1]: https://www.postgresql.org/message-id/CAAAe_zAJz1nP3h2vHNi%2BwgN_cmzRcNpSHrUj8tRyP%2BVPexUwcg%40mail.gmail.com
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