Using a variable on a PostgreSQL function to drop a schema - postgresql

Question

I'm trying to create a function on PostgreSQL, and I have some problem to use a local variable. Here's my code :
DECLARE query RECORD;
DECLARE schema_name TEXT;
BEGIN
FOR query IN SELECT * FROM context WHERE created_at + make_interval(days => duration) <= CURRENT_TIMESTAMP LOOP
SELECT lower(quote_ident(query.title)) INTO schema_name;
DROP SCHEMA schema_name CASCADE;
DELETE FROM context WHERE id = query.id;
END LOOP;
RETURN 1;
END;
$$ LANGUAGE plpgsql;
The select and delete queries work fine, and I've made a test returning the value of schema_name variable, and it's OK.
My problem is with this line :
DROP SCHEMA schema_name CASCADE;
I get an error as "the schema 'schema_name' doesn't exist".
I'd really appreciate any ideas for how to use this variable to do the drop query.

Answer 1

You need dynamic SQL for this:
DECLARE
query RECORD;
BEGIN
FOR query IN SELECT id, lower(title) as title
FROM context
WHERE created_at + make_interval(days => duration) <= CURRENT_TIMESTAMP
LOOP
execute format('DROP SCHEMA %I CASCADE', query.title);
DELETE FROM context WHERE id = query.id;
END LOOP;
RETURN 1;
END;
$$ LANGUAGE plpgsql;
I also removed the unnecessary SELECT statement to make the title lower case, this is better done in the query directly.
Also: variable assignment is faster with := then with select, so:
schema_name := lower(quote_ident(query.title));
would be better if the variable was needed.