Postgres 13

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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 t (x INT, y INT);
ALTER TABLE t ADD CONSTRAINT anyabel CHECK (x IS NULL OR y IS NULL)

CREATE TABLE

ALTER TABLE

INSERT INTO t VALUES (NULL,42);

INSERT 0 1

INSERT INTO t VALUES (42,42);

ERROR:  new row for relation "t" violates check constraint "anyabel"
DETAIL:  Failing row contains (42, 42).