Use an empty query instead of SELECT 1

Using an empty query skips the PG query planner so it is more efficient

activerecord/CHANGELOG.md

@@ -1,3 +1,9 @@
+*   Use an empty query to check if the PostgreSQL connection is still active
+
+    An empty query is faster than `SELECT 1`.
+
+    *Heinrich Lee Yu*
+
 *   Add `ActiveRecord::Base#previously_persisted?`
 
     Returns `true` if the object has been previously persisted but now it has been deleted.

activerecord/lib/active_record/connection_adapters/postgresql_adapter.rb

@@ -292,7 +292,7 @@ def self.database_exists?(config)
       # Is this connection alive and ready for queries?
       def active?
         @lock.synchronize do
-          @connection.query "SELECT 1"
+          @connection.query ";"
         end
         true
       rescue PG::Error