Wrong results: WHERE on inheritance parent column drops all rows when cross-joined with a distributed table through LEFT JOIN ... ON FALSE

[mrigger]

Wrong results: WHERE on inheritance parent column drops all rows when cross-joined with a distributed table through LEFT JOIN ... ON FALSE

Summary

When a regular table that has an inheritance child is cross-joined with a distributed table and combined with LEFT JOIN ... ON FALSE, a WHERE predicate on a column of the inheritance parent always evaluates to false on the worker, so the query returns zero rows.

Versions affected

• Citus 13.0 (citusdata/citus:13.0-pg16)
• Citus 14.0 (citusdata/citus:postgres_17)
• Citus nightly — extension 15.0-1 on PostgreSQL 18.1

Vanilla PostgreSQL 17 (clean postgres:17 image, shared_preload_libraries empty, citus extension not installed) returns the correct answer.

Repro

CREATE TABLE t0(c0 REAL);
CREATE TABLE t1() INHERITS(t0);
CREATE TABLE t3(c0 REAL);
SELECT create_distributed_table('t3', 'c0');
INSERT INTO t1(c0) VALUES (0.5);
INSERT INTO t3(c0) VALUES (1);

SELECT 1 FROM t3, t0 LEFT JOIN t1 ON FALSE;                          -- returns 1 (correct)
SELECT 1 FROM t3, t0 LEFT JOIN t1 ON FALSE WHERE t0.c0 IS NOT NULL;  -- returns 0 (expected: 1)
SELECT 1 FROM t3, t0 LEFT JOIN t1 ON FALSE WHERE TRUE;               -- returns 1 (correct)

t0.c0 is 0.5 (inherited from t1), so the second query should return one row.

Diagnosis

EXPLAIN (ANALYZE, VERBOSE) of the failing query shows the worker SQL Citus emits:

SELECT 1
FROM (SELECT NULL::real AS c0 WHERE false) t3(c0),
     ( (SELECT NULL::real AS c0
        FROM (SELECT intermediate_result."dummy-1"
              FROM read_intermediate_result('1_1','binary')
                   intermediate_result("dummy-1" integer)) t0_1) t0
       LEFT JOIN
       (SELECT NULL::real AS c0
        FROM (SELECT intermediate_result."dummy-1"
              FROM read_intermediate_result('1_2','binary')
                   intermediate_result("dummy-1" integer)) t1_1) t1
       ON (false) )
WHERE (t0.c0 IS NOT NULL)
->  Result   One-Time Filter: false

t0.c0 is rewritten to a literal NULL::real, so WHERE (NULL IS NOT NULL) constant-folds to false. The real values are pulled into intermediate_result but stay buried in the inner sub-select that nothing references.

Trigger conditions (each removal makes the bug disappear): inheritance child of t0, the cross-joined t3 being distributed, the LEFT JOIN ... ON FALSE, and a WHERE referencing the inheritance parent's column.

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This bug was analyzed with the help of Claude.