Postgres 13

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-- meta tables for schema and table name    
CREATE TABLE schma(schma_id int, schma text);
INSERT INTO  schma VALUES (1, 'public');

CREATE TABLE tbl(tbl_id int, tbl text);
INSERT INTO  tbl VALUES (1, 't1'), (2, 't2');

-- dummy tables to be used in example query:
CREATE TABLE t1(id int);
INSERT INTO  t1 VALUES (1),(2);

CREATE TABLE t2(foo text);
INSERT INTO  t2 VALUES ('some text'), ('some more text');

CREATE TABLE

INSERT 0 1

CREATE TABLE

INSERT 0 2

CREATE TABLE

INSERT 0 2

CREATE TABLE

INSERT 0 2

CREATE OR REPLACE FUNCTION f_dynaquery(_schma_id int
                                     , _tbl_id int
                                     , _col text
                                     , _type anyelement)
  RETURNS TABLE(col anyelement)
  LANGUAGE plpgsql AS
$func$
BEGIN
   RETURN QUERY EXECUTE format(
      'SELECT %I FROM %I.%I'
    , _col
    , (SELECT schma FROM schma WHERE schma_id = _schma_id)
    , (SELECT tbl   FROM tbl   WHERE tbl_id   = _tbl_id)
   );
END
$func$;

COMMENT ON FUNCTION f_dynaquery(int, int, text, anyelement)
IS 'Query any column from a dynamically assembled tablename.
$1 .. id of schema
$2 .. id of table
$3 .. name of column
$4 .. type of column (only data type matters, not the value)';

CREATE FUNCTION

COMMENT

SELECT col FROM f_dynaquery(1, 1, 'id', NULL::int);

col

1

2

SELECT 2

SELECT col FROM f_dynaquery(1, 2, 'foo', NULL::text);

col

some text

some more text

SELECT 2

CREATE OR REPLACE FUNCTION f_dynaquery_old(int, int, _col text, _type anyelement, OUT col anyelement)
  RETURNS SETOF anyelement
  LANGUAGE plpgsql AS
$func$
BEGIN
   RETURN QUERY EXECUTE '
   SELECT ' || quote_ident(_col) || '
   FROM   ' || (
       (SELECT schma FROM schma WHERE schma_id = $1) || '.' ||
       (SELECT tbl   FROM tbl   WHERE tbl_id   = $2))::regclass;
END
$func$;

CREATE FUNCTION

SELECT col FROM f_dynaquery_old(1, 1, 'id', NULL::int);

col

1

2

SELECT 2

SELECT col FROM f_dynaquery_old(1, 2, 'foo', NULL::text);

col

some text

some more text

SELECT 2