How do I get the primary key(s) of a table from Postgres via plpgsql?

The query above is very bad as it is really slow.

I would recommend this official version:

http://wiki.postgresql.org/wiki/Retrieve_primary_key_columns

if schema is needed the query is as follows

SELECT               
  pg_attribute.attname, 
  format_type(pg_attribute.atttypid, pg_attribute.atttypmod) 
FROM pg_index, pg_class, pg_attribute, pg_namespace 
WHERE 
  pg_class.oid = 'foo'::regclass AND 
  indrelid = pg_class.oid AND 
  nspname = 'public' AND 
  pg_class.relnamespace = pg_namespace.oid AND 
  pg_attribute.attrelid = pg_class.oid AND 
  pg_attribute.attnum = any(pg_index.indkey)
 AND indisprimary

To provide a straight bit of SQL, you can list the primary key columns and their types with:

SELECT c.column_name, c.data_type
FROM information_schema.table_constraints tc 
JOIN information_schema.constraint_column_usage AS ccu USING (constraint_schema, constraint_name) 
JOIN information_schema.columns AS c ON c.table_schema = tc.constraint_schema
  AND tc.table_name = c.table_name AND ccu.column_name = c.column_name
WHERE constraint_type = 'PRIMARY KEY' and tc.table_name = 'mytable';

The following SQL statement works for me:

SELECT a.attname
FROM   pg_index i
JOIN   pg_attribute a ON a.attrelid = i.indrelid
                     AND a.attnum = ANY(i.indkey)
WHERE  i.indrelid = 'tablename'::regclass
AND    i.indisprimary;

It is directly taken from here.