Postgres 11

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Help with an interesting Postgres question: Why isn't an Index Only Scan used on a partition accessed via the parent table?.

CREATE TABLE tab(a INT, b INT)
INSERT INTO tab VALUES (17,-1)
1 rows affected

WITH cte(i, list) AS (
  VALUES (23, ARRAY[1]),
   (20, ARRAY[0]),
   (17, ARRAY[-1,0,1]),
   (55, ARRAY[-1, 0])
)
SELECT *
FROM tab t
JOIN cte c
  ON t.a = c.i
 AND t.b = ANY(list)


a
b
i
list


17
-1
17
{-1,0,1}