Select for loop in postgres - postgresql

Question

I want to get the count of benches I have overall. I have multiple schema as classroom and each clsssroom has multiple tables one of them is benches. I want to count number of benches I have in all the schema. So its like classroom A has 6-chairs and 7-benches Classroom B has 2-chairs and 8 benches so I have 15 benches.
I have two queries already
SELECT classroom_name FROM public.classroom
SELECT COUNT(DISTINCT id) FROM <classroom_name>.benches;
I want to get all the values from query 1 and loop them in query 2. Solution I came close to is
CREATE OR REPLACE FUNCTION all_customers_dynamic() RETURNS SETOF classroom AS $$
DECLARE
schema RECORD;
BEGIN
FOR schema IN EXECUTE
format(
'SELECT classroom_name FROM public.classroom'
)
LOOP
RETURN QUERY EXECUTE
format('SELECT COUNT(DISTINCT id) FROM %I.benches;', schema.classroom_name);
END LOOP;
END;
$$ LANGUAGE plpgsql;

Answer 1

Sample Data for testing:
CREATE SCHEMA classroom1;
CREATE SCHEMA classroom2;
CREATE TABLE classroom1.benches (
id int4 NULL
);
INSERT INTO classroom1.benches (id) VALUES(1);
INSERT INTO classroom1.benches (id) VALUES(2);
INSERT INTO classroom1.benches (id) VALUES(3);
INSERT INTO classroom1.benches (id) VALUES(4);
INSERT INTO classroom1.benches (id) VALUES(5);
INSERT INTO classroom1.benches (id) VALUES(6);
CREATE TABLE classroom2.benches (
id int4 NULL
);
INSERT INTO classroom2.benches (id) VALUES(1);
INSERT INTO classroom2.benches (id) VALUES(2);
INSERT INTO classroom2.benches (id) VALUES(3);
INSERT INTO classroom2.benches (id) VALUES(4);
INSERT INTO classroom2.benches (id) VALUES(5);
INSERT INTO classroom2.benches (id) VALUES(6);
INSERT INTO classroom2.benches (id) VALUES(7);
INSERT INTO classroom2.benches (id) VALUES(8);
CREATE TABLE public.classroom (
classroom_name varchar NULL
);
INSERT INTO public.classroom (classroom_name) VALUES('classroom1');
INSERT INTO public.classroom (classroom_name) VALUES('classroom2');
You have two options for do it.
Get benches tables list from your public.classroom table
CREATE OR REPLACE FUNCTION all_customers_dynamic()
RETURNS int4
AS $$
DECLARE
sh RECORD;
k int4;
total int4;
begin
total = 0;
for sh in execute
'SELECT classroom_name FROM public.classroom'
loop
execute 'select count(distinct id) from ' || sh.classroom_name || '.benches' into k;
total = total + k;
end loop;
return total;
END;
$$ LANGUAGE plpgsql;
Get all benches named tables automatically from information_schema (system tables)
CREATE OR REPLACE FUNCTION all_customers_dynamic_1()
RETURNS int4
AS $$
DECLARE
sh RECORD;
k int4;
total int4;
begin
total = 0;
for sh in execute
'SELECT schemaname || ''.'' || tablename as tb from pg_catalog.pg_tables where tablename = ''benches'''
loop
execute 'select count(distinct id) from ' || sh.tb into k;
total = total + k;
end loop;
return total;
END;
$$ LANGUAGE plpgsql;