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 test
(
    id int primary key,
    foo bool
);

CREATE TABLE


CREATE UNIQUE INDEX only_one_row_with_column_true_uix 
    ON test (foo) WHERE (foo);  --> where foo is true

CREATE INDEX


insert into test values (1, false);
insert into test values (2, true);
insert into test values (3, false);
insert into test values (4, false);
insert into test values (5, true);

ERROR:  duplicate key value violates unique constraint "only_one_row_with_column_true_uix"
DETAIL:  Key (foo)=(t) already exists.