[YSQL] DISTINCT over UNION ALL or partitioned table fails with “could not find pathkey item to sort”

  CREATE TABLE t1 (a INT);
  CREATE TABLE t2 (a INT);
  CREATE INDEX t1_a_idx ON t1 (a ASC);
  CREATE INDEX t2_a_idx ON t2 (a ASC);

  INSERT INTO t1 SELECT i % 2 FROM generate_series(1, 1000) i;
  INSERT INTO t2 SELECT i % 3 FROM generate_series(1, 1000) i;

  ANALYZE t1;
  ANALYZE t2;

  SELECT DISTINCT x
  FROM (
    SELECT a FROM t1
    UNION ALL
    SELECT a FROM t2
  ) s(x)
  ORDER BY 1;

  Actual result

  ERROR: could not find pathkey item to sort

  Expected result

  0
  1
  2

yugabyte=# CREATE TABLE pt(k INT, v INT) PARTITION BY RANGE (k);
CREATE TABLE
yugabyte=# CREATE TABLE pt1 PARTITION OF pt FOR VALUES FROM (0) TO (5);
CREATE TABLE
yugabyte=# CREATE TABLE pt2 PARTITION OF pt FOR VALUES FROM (5) TO (10);
CREATE TABLE
yugabyte=# CREATE INDEX pt_v_idx ON pt (v ASC);
CREATE INDEX
yugabyte=# INSERT INTO pt (SELECT i%10, i%3 FROM GENERATE_SERIES(1, 100) AS i);
INSERT 0 100
yugabyte=# 
yugabyte=# EXPLAIN (COSTS OFF) SELECT DISTINCT v FROM pt ORDER BY 1;
ERROR:  could not find pathkey item to sort