Postgres 10

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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 or replace function test(json_in json) returns TABLE( s text) as
$$ 
 begin
  return  query select j.s from json_array_elements_text(json_in) as j(s);
end $$ language plpgsql;

CREATE FUNCTION

select  test('["a","b","c"]')

test

a

b

c

SELECT 3

select '["a","b","c"]'::json[]

ERROR:  malformed array literal: "["a","b","c"]"
LINE 1: select '["a","b","c"]'::json[]
               ^
DETAIL:  "[" must introduce explicitly-specified array dimensions.