Re: support parameterized (LATERAL) foreign joins

Hi
Could you please rebase ? it's not applied.

On Fri, Oct 2, 2026 at 10:45 PM vaibhave postgres <
postgresvaibhave(at)gmail(dot)com> wrote:

> Hi hackers,
>
> create_foreign_join_path() currently rejects any parameterized foreign
> join outright:
>
> ```
> if (!bms_is_empty(required_outer) ||
> !bms_is_empty(rel->lateral_relids))
> elog(ERROR, "parameterized foreign joins are not supported yet");
> ```
>
> and postgres_fdw's postgresGetForeignJoinPaths() side-steps that by
> declining to push down any join whose relation has lateral references:
>
> ```
> /* This code does not work for joins with lateral references ... */
> if (!bms_is_empty(joinrel->lateral_relids))
> return;
> ```
>
> For example in the below query (foreign i1/i2, local o):
>
> SELECT o.f1, ss.x
> FROM local_tbl o,
> LATERAL (SELECT 1 AS x
> FROM (SELECT o.f1 AS lat, i2.f1 AS loc
> FROM int8_tbl i1, int4_tbl i2) ss1
> RIGHT JOIN int4_tbl i3 ON (i3.f1 > 1)
> WHERE ss1.loc = ss1.lat) ss;
>
> The RIGHT JOIN here keeps the LATERAL reference from being flattened into
> an ordinary join clause, so the inner foreign join (i1 x i2) really does
> require a parameterized path and is never pushed down.
>
> The attached patch teaches the planner to build such paths and as an
> example to postgres_fdw.
>
> create_foreign_join_path() no longer errors on a parameterized request.
> Instead it builds a ParamPathInfo via a new helper,
>
> get_joinrel_parampathinfo_pushdown(joinrel, required_outer,
> restrict_clauses, rows)
>
> in relnode.c. The FDW does not have a pair of input paths
> to describe how the join is formed, so instead it hands us the
> parameterization, the estimated rowcount, and the RestrictInfos the path
> will enforce.
>

--
Regards,
Nurlan