Re: support parameterized (LATERAL) foreign joins

Rebased onto master, thanks

On Sat, Oct 3, 2026 at 1:19 AM Nurlan Tulemisov <nurlan(dot)tulemisov(at)gmail(dot)com>
wrote:

> 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
>