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Thread: BUG #18852: Unexpected expression in subquery output

BUG #18852: Unexpected expression in subquery output

From
PG Bug reporting form
Date:
The following bug has been logged on the website:

Bug reference:      18852
Logged by:          Robins Tharakan
Email address:      tharakan@gmail.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;

- The issue surfaced after commit 824dbea3e41efa3b35094163c834988dea7eb139
[1]
Testing for start crashing
Checking (682c5be25c2~0) - 682c5be25c - fail
Checking (682c5be25c2~10) - 2b73a8cd33 - fail
Checking (682c5be25c2~30) - 3691edfab9 - fail
.
.
Checking (682c5be25c2~3470) - 278eb13c48 - pass
Checking (682c5be25c2~3450) - 7db01fbcef - fail
Checking (682c5be25c2~3460) - 719b342d36 - fail
Checking (682c5be25c2~3465) - c2a465b2c9 - fail
Checking (682c5be25c2~3467) - 7636725b92 - fail
Checking (682c5be25c2~3468) - bfc677c3bc - fail
Checking (682c5be25c2~3469) - 824dbea3e4 - fail
Surfacing Commit is 824dbea3e41efa3b35094163c834988dea7eb139

-
robins

Reference:
1.
https://git.postgresql.org/gitweb/?p=postgresql.git;a=commitdiff;h=824dbea3e41efa3b35094163c834988dea7eb139

Re: BUG #18852: Unexpected expression in subquery output

From
Tender Wang
Date:


PG Bug reporting form <noreply@postgresql.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@gmail.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

Re: BUG #18852: Unexpected expression in subquery output

From
Robins Tharakan
Date:

On Tue, 18 Mar 2025 at 01:51, Tender Wang <tndrwang@gmail.com> wrote:

PG Bug reporting form <noreply@postgresql.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


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;


FWIW, there's additional discussion happening in the original thread -