how to convert string as sql syntax - postgresql

Question

An example what I am attempting to do,
SELECT count(*) as "count"
FROM (
SELECT overlay('db_xx.company_summary' placing 'US' from 4)
) as s
This returns count to be 1, I would expect it to count all rows in db_us.company_summary 1*10^6
I would expect this to result in a query to similar to this;
SELECT count(*) as "count"
FROM (db_us.company_summary')
I attempted the overlay function, to be similar to the above query. I am not sure if its possible to do this in SQL.
Normally in python you would do something like this;
"hello {}".format("world")
So I would like the inputted string to act as a SQL syntax command.

Answer 1

Plain SQL does not allow to parameterize identifiers (or anything but values). You need dynamic SQL. Examples:
Truncating all tables in a Postgres database
How to use variable as table name in plpgsql
How do I use variables in a postgresql function for loop query
Something like this:
Minimal test setup (should be in the question):
CREATE TABLE country_code (iso2 text PRIMARY KEY);
INSERT INTO country_code VALUES ('US'), ('GB'), ('AT');
CREATE SCHEMA db_us;
CREATE SCHEMA db_gb;
CREATE SCHEMA db_at;
CREATE TABLE db_us.company_summary (foo int);
CREATE TABLE db_gb.company_summary (foo int);
CREATE TABLE db_at.company_summary (foo int);
INSERT INTO db_us.company_summary VALUES (1), (2);
INSERT INTO db_gb.company_summary VALUES (1), (2), (3);
INSERT INTO db_at.company_summary VALUES (1), (2), (3), (4);
PL/pgSQL function with dynamic SQL:
CREATE OR REPLACE FUNCTION f_counts()
RETURNS SETOF bigint
LANGUAGE plpgsql AS
$func$
DECLARE
_lower_iso2 text;
_ct bigint;
BEGIN
FOR _lower_iso2 IN
SELECT lower(iso2) FROM country_code
LOOP
RETURN QUERY EXECUTE
format ('SELECT count(*) AS count FROM %I.company_summary'
, overlay('db_xx' PLACING _lower_iso2 FROM 4)
);
END LOOP;
END
$func$;
Call:
SELECT * FROM f_counts();
Result:
f_counts
---------
2
3
4
db<>fiddle here
Be aware that Postgres identifiers are case sensitive. See:
Are PostgreSQL column names case-sensitive?