X-Git-Url: https://git.arvados.org/arvados.git/blobdiff_plain/44ff73fa397095d69819761e66933783a5f6d541..443f3228eb4c56849f77ae9c421dd1cc6fdbc5f1:/services/api/lib/load_param.rb

diff --git a/services/api/lib/load_param.rb b/services/api/lib/load_param.rb
index 718aaeaf69..d7b9bb7513 100644
--- a/services/api/lib/load_param.rb
+++ b/services/api/lib/load_param.rb
@@ -111,7 +111,31 @@ module LoadParam
     # (e.g., [] or ['owner_uuid desc']), fall back on the default
     # orders to ensure repeating the same request (possibly with
     # different limit/offset) will return records in the same order.
-    @orders += model_class.default_orders
+    #
+    # Clean up the resulting list of orders such that no column
+    # uselessly appears twice (Postgres might not optimize this out
+    # for us) and no columns uselessly appear after a unique column
+    # (Postgres does not optimize this out for us; as of 9.2, "order
+    # by id, modified_at desc, uuid" is slow but "order by id" is
+    # fast).
+    orders_given_and_default = @orders + model_class.default_orders
+    order_cols_used = {}
+    @orders = []
+    orders_given_and_default.each do |order|
+      otablecol = order.split(' ')[0]
+
+      next if order_cols_used[otablecol]
+      order_cols_used[otablecol] = true
+
+      @orders << order
+
+      otable, ocol = otablecol.split('.')
+      if otable == table_name and model_class.unique_columns.include?(ocol)
+        # we already have a full ordering; subsequent entries would be
+        # superfluous
+        break
+      end
+    end
 
     case params[:select]
     when Array