PostgreSQL: Using schema and table names from other tables?

Question

Consider the situation where I have all my schema names in one table and all my table names in another table.

Is it possible to do something like in the following?

SELECT value FROM (SELECT schema_name FROM schemas).(SELECT table_name FROM tables)

Or do I have to break up the query into three SELECT's?

Answer 1

Jack has demonstrated the way to go. However, I feel there is room for improvement:

DROP SCHEMA x CASCADE;
CREATE SCHEMA x;

CREATE TABLE x.t1(id int);
INSERT INTO  x.t1 VALUES (1),(2);

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

CREATE TABLE x.schma(schma_id int, schma text);
INSERT INTO  x.schma VALUES (1, 'x');

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

CREATE OR REPLACE FUNCTION x.f_dynaquery(int, int, _col text, _type anyelement, OUT col anyelement)
  RETURNS SETOF anyelement AS
$body$
BEGIN

RETURN QUERY EXECUTE '
SELECT ' || quote_ident(_col) || '
FROM   ' || (
    (SELECT schma FROM x.schma WHERE schma_id = $1)
     || '.' ||
    (SELECT tbl   FROM x.tbl   WHERE tbl_id   = $2))::regclass;

END;
$body$
  LANGUAGE plpgsql STABLE;

COMMENT ON FUNCTION x.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 (actual type matters, not the values)';

Call:

SELECT col FROM x.f_dynaquery(1, 1, 'id', NULL::int);
 col
-----
   1
   2
(2 rows)

SELECT col FROM x.f_dynaquery(1, 2, 'foo', NULL::text);
  col
----------------
 some text
 some more text
(2 rows)

Major points

• Function can return any column of any type. Read in the manual about Polymorphic Types and Declaring Function Parameters
• Safeguard against SQL injection by using quote_ident() and casting to regclass.
• Retrieve the tablename by query as OP requested. Example is query by id but anything is possible.
• Name the returned column, so it is easier to reference.

Answer 2

You need dynamic SQL - perhaps something like this:

create role stack;
create schema authorization stack;
set role stack;

create or replace function f(p_schema in text, p_table in text)
                           returns setof integer language plpgsql immutable as $$
begin
  return query execute 'select value from '||p_schema||'.'||p_table;
end;$$;

create table t1(value integer);
insert into t1(value) values (1);
insert into t1(value) values (2);

create table t2(value integer);
insert into t2(value) values (1);

create table schemas(schema_name text);
insert into schemas(schema_name) values ('stack');

create table tables(table_name text);
insert into tables(table_name) values ('t1');
insert into tables(table_name) values ('t2');
insert into tables(table_name) values ('t1');
insert into tables(table_name) values ('t2');

select f(schema_name, table_name) from schemas cross join tables;
 f
---
 1
 2
 1
(3 rows)

I'm assuming that every table is present in every schema as the question implies