Fix primary keys retrieval query (#198)

The previous query had some issues when used under clusters with many
schema objects, this one improves query performance

pkg/state/state.go

@@ -143,20 +143,20 @@ BEGIN
 					) c
 				),
 				'primaryKey', (
-				  SELECT json_agg(kcu.column_name) AS primary_key_columns
-				  FROM information_schema.table_constraints AS tc 
-				  JOIN information_schema.key_column_usage AS kcu 
-				  ON tc.constraint_name = kcu.constraint_name
-				  WHERE tc.table_name = t.relname
-				  AND tc.table_schema = schemaname
-				  AND tc.constraint_type = 'PRIMARY KEY'
+					SELECT json_agg(pg_attribute.attname) AS primary_key_columns
+					FROM pg_index, pg_attribute
+					WHERE
+						indrelid = t.oid AND
+						nspname = schemaname AND
+						pg_attribute.attrelid = t.oid AND
+						pg_attribute.attnum = any(pg_index.indkey)
+						AND indisprimary
 				),
 				'indexes', (
 				  SELECT json_object_agg(pi.indexrelid::regclass, json_build_object(
 				    'name', pi.indexrelid::regclass
 				  ))
 				  FROM pg_index pi 
-				  INNER JOIN pg_class pgc ON pi.indexrelid = pgc.oid
 				  WHERE pi.indrelid = t.oid::regclass
 				)
 			)) FROM pg_class AS t