Postgres 9.5

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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 taxrate (
   taxrate int PRIMARY KEY
 , standard bool DEFAULT true
 , CONSTRAINT standard_true_or_null CHECK (standard) -- yes, that's the whole constraint
 , CONSTRAINT standard_only_1_true UNIQUE (standard)
);
CREATE TABLE

INSERT INTO taxrate (taxrate) VALUES (42);  -- becomes standard since not explicitly not the standard
INSERT 0 1

INSERT INTO taxrate (taxrate, standard) VALUES (1, NULL),  (2, NULL);
INSERT 0 2

INSERT INTO taxrate (taxrate) VALUES (43);  -- Fails.
ERROR:  duplicate key value violates unique constraint "standard_only_1_true"
DETAIL:  Key (standard)=(t) already exists.


BEGIN;
UPDATE taxrate SET standard = NULL WHERE standard;
UPDATE taxrate SET standard = TRUE WHERE taxrate = 2;
COMMIT;
COMMIT

--  Fails:
WITH kingdead AS (
   UPDATE taxrate
   SET standard = NULL
   WHERE standard
   )
UPDATE taxrate
SET standard = TRUE
WHERE taxrate = 1;

-- would be  possibly with a more expensive DEFERRABLE INITIALLY IMMEDIATE constraint
ERROR:  duplicate key value violates unique constraint "standard_only_1_true"
DETAIL:  Key (standard)=(t) already exists.