PostgreSQL: Return JSON from function using language plpgsql

Question

I have the following table with two fields:

create table tbl_jtest
(
cola int,
colb varchar(10)
);

Inserting some records:

insert into tbl_jtest values(1,'a');
insert into tbl_jtest values(2,'b');
insert into tbl_jtest values(3,'c');
insert into tbl_jtest values(4,'d');

Function:

CREATE OR REPLACE FUNCTION ufn_jtest1(pcola int) 
RETURNS json AS
$$
BEGIN
IF pcola = 1 
THEN
    RETURN QUERY SELECT to_json(a.cola) FROM tbl_jtest a;
ELSE
    RETURN QUERY  SELECT to_json(a.colb) FROM tbl_jtest a;
END IF;
END;
$$ LANGUAGE plpgsql;

Error details:

ERROR: cannot use RETURN QUERY in a non-SETOF function LINE 7: RETURN QUERY SELECT to_json(a.cola) FROM tbl_jtest a; ^

I have tried the followings:

Try 1:

PERFORM to_json(a.cola) FROM tbl_jtest a;

Try 2:

RETURN QUERY PERFORM to_json(a.cola) FROM tbl_jtest a;

Answer 1

Or are you looking for something like this?

create table tbl_jtest
(
cola int,
colb varchar(10),
colc varchar(10)

);

insert into tbl_jtest values(1,'a','e');
insert into tbl_jtest values(2,'b','f');
insert into tbl_jtest values(3,'c','g');
insert into tbl_jtest values(4,'d','h');


SELECT * FROM tbl_jtest;

CREATE OR REPLACE FUNCTION ufn_jtest1(pcola int) 
RETURNS table (j json) AS
$$
BEGIN
IF pcola = 1 
THEN
    RETURN QUERY  SELECT row_to_json(a) FROM (SELECT cola, colb FROM tbl_jtest) a;
ELSE
    RETURN QUERY  SELECT to_json(a) FROM (SELECT colb, colc FROM tbl_jtest) a;
END IF;
END;
$$ LANGUAGE plpgsql;

Test 1

SELECT ufn_jtest1(1);

Output 1

    ufn_jtest1
1   {"cola":1,"colb":"a"}
2   {"cola":2,"colb":"b"}
3   {"cola":3,"colb":"c"}
4   {"cola":4,"colb":"d"}

Test2

SELECT ufn_jtest1(2);

Output2 ufn_jtest1

1   {"colb":"a","colc":"e"}
2   {"colb":"b","colc":"f"}
3   {"colb":"c","colc":"g"}
4   {"colb":"d","colc":"h"}