CREATE OR REPLACE FUNCTION YOUR_SCHEMA.fn_returnalldata() 
RETURNS jsonb AS $$
DECLARE 
    result jsonb := '{}'::jsonb;
    table_name text;
    table_json jsonb;
BEGIN 
    FOR table_name IN
        SELECT tablename
        FROM pg_tables
        WHERE schemaname = 'YOUR_SCHEMA' 
    LOOP

        EXECUTE format(
            '
            SELECT jsonb_build_object(%L, (SELECT row_to_json(row) FROM (
                SELECT * 
                FROM YOUR_SCHEMA.%I 
                LIMIT 1
            ) row))',
            table_name,
            table_name
        ) INTO table_json;

        result := result || table_json;
    END LOOP;

    RETURN result;
END;
$$ LANGUAGE plpgsql;


select * from YOUR_SCHEMA.fn_returnalldata()