Use ntuples to populate row_count instead of count for Postgres

PG::Result#ntuples uses a libpq function for getting the number of rows returned from a query, whereas #count is provided by Enumerable and thus iterates through the entire result set.

Author: jcalvert

activerecord/CHANGELOG.md

@@ -1,3 +1,7 @@
+*   Use ntuples to populate row_count instead of count for Postgres
+
+    *Jonathan Calvert*
+
 *   Fix checking whether an unpersisted record is `include?`d in a strictly
     loaded `has_and_belongs_to_many` association.
 

activerecord/lib/active_record/connection_adapters/postgresql/database_statements.rb

@@ -165,7 +165,7 @@ def perform_query(raw_connection, sql, binds, type_casted_binds, prepare:, notif
             verified!
 
             notification_payload[:affected_rows] = result.cmd_tuples
-            notification_payload[:row_count] = result.count
+            notification_payload[:row_count] = result.ntuples
             result
           end