Re: BUG #18852: Unexpected expression in subquery output

PG Bug reporting form <noreply(at)postgresql(dot)org> 于2025年3月17日周一 16:35写道：

> The following bug has been logged on the website:
>
> Bug reference: 18852
> Logged by: Robins Tharakan
> Email address: tharakan(at)gmail(dot)com
> PostgreSQL version: Unsupported/Unknown
> Operating system: Ubuntu
> Description:
>
> Hi,
>
> The following SQL returns this error:
>
> $ psql -f repro.sql >/dev/null
> psql:repro.sql:11: ERROR: unexpected expression in subquery output
>
> $ cat repro.sql
> CREATE SCHEMA a;
> CREATE SCHEMA b;
> CREATE EXTENSION postgres_fdw ;
> CREATE SERVER d FOREIGN DATA WRAPPER postgres_fdw;
> CREATE FOREIGN TABLE a.e () SERVER d;
> CREATE FOREIGN TABLE a.f (g text) SERVER d;
> CREATE FOREIGN TABLE b.h () SERVER d;
> SELECT FROM a.f RIGHT JOIN b.h ON EXISTS (SELECT FROM a.e WHERE (SELECT
> NULL) > g) LIMIT 9;
>

I can reproduce this issue on HEAD.
SELECT FROM a.f RIGHT JOIN b.h ON EXISTS (SELECT FROM a.e WHERE (SELECT
NULL) > g) LIMIT 9;

The join order of the above query is b.h left join (a.f semi join a.e). The
NULL > g is pushdown to a.f rel.
When we process semijoin, the (NULL > g) is added into
fpinfo->remote_conds, then it is added into the finfo->joinclauses
when we process the left join.

So in deparseFromExprForRel(), we should append the fpinfo->joinclauses to
buf, then we will enter get_relation_column_alias_ids(node, innerrel,
relno, colon)
in is_subquery_var(). The innerrel is joinrel of a.f and a.e, the node is
var (g), but innerrel->reltarget->expr is NIL. So the error triggers.

The (null > g) should not be the leftjoin joinclauses, if I understand
correctly. And I see the comments in foreign_join_ok():
...
else if (jointype == JOIN_LEFT || jointype == JOIN_RIGHT || jointype ==
JOIN_FULL)
{
/*
* Conditions, generated from semi-joins, should be evaluated before
* LEFT/RIGHT/FULL join.
*/
...

I add below code in foreign_join_ok() if jointype is LEFT:
case JOIN_LEFT:
if (bms_is_empty(fpinfo_i->hidden_subquery_rels))
fpinfo->joinclauses = list_concat(fpinfo->joinclauses,

fpinfo_i->remote_conds);

Then I get the plan like below:
postgres=# explain verbose SELECT FROM a.f RIGHT JOIN b.h ON EXISTS (SELECT
FROM a.e WHERE (SELECT
NULL) > g) limit 1;

QUERY PLAN

-------------------------------------------------------------------------------------------------------------------------------------------------------------------------
Foreign Scan (cost=100.01..100.21 rows=1 width=0)
Relations: (b.h) LEFT JOIN ((a.f) SEMI JOIN (a.e))
Remote SQL: SELECT NULL FROM (b.h r2 LEFT JOIN (SELECT NULL FROM a.f r1
WHERE (($1::text > r1.g)) AND EXISTS (SELECT NULL FROM a.e r4)) s5 ON
(TRUE)) LIMIT 1::bigint
InitPlan 1
-> Result (cost=0.00..0.01 rows=1 width=32)
Output: NULL::text
(6 rows)

I'm not very familiar with fdw codes. Any thoughts?
--
Thanks,
Tender Wang