Query is not working in Postgresql function [duplicate] - postgresql

Question

I have 30 state wise data tables. Table name like aa_shg_detail, ab_shg_detail, ac_shg_detail.
I have also main state table in which state short names and state codes are stored. I have created 2 postgresql functions getTableName(Code text) and getDataByTable().
In the first function I pass the state code so it fetches the state short name and short name concat with _shg_detail String and prepare full table name and return it. Example: If I pass state code 2 the query fetch state short name based on state code 2 from the state's main table. The state short name is 'ab' for state code 2 so after concat state short name with _shg_detail first function return ab_shg_detail table name.
Second function gets the table name from first function and fetch data from that table. But I am getting error in the second function.
CREATE OR REPLACE FUNCTION getTableName(code text)
RETURNS text
AS $$
select concat(lower(state_short_name), '_shg_detail') from main_state where state_code = code))
$$
LANGUAGE sql;
CREATE OR REPLACE FUNCTION getDataByTable()
RETURNS text AS $$
DECLARE
tablename text;
BEGIN
tablename := gettablename('2');
RETURN (select shg_code from tablename);
END;
$$ LANGUAGE plpgsql;
When I execute a second function select getDataByTable() then I am getting this error every time:
ERROR: relation "tablename" does not exist
LINE 1: SELECT (select shg_code from tableName)

Answer 1

You need dynamic SQL for that:
CREATE OR REPLACE FUNCTION getDataByTable()
RETURNS text AS $$
DECLARE
tablename text;
l_result text;
BEGIN
tablename := gettablename('2');
execute format('select shg_code from %I', tablename)
into l_result;
RETURN l_result;
END;
$$ LANGUAGE plpgsql;
The %I placeholder of the format() function properly deals with quoting of identifiers if needed.