Ticket #9870: aggregation-m2m-opt.3.diff

django/db/models/sql/query.py

@@ -1194 +1194 @@
 
         elif (len(field_list) > 1 or
             field_list[0] not in [i.name for i in opts.fields]):
+                
+            join_alias = self.get_initial_alias()
+            
             field, target, opts, join_list, last, _ = self.setup_joins(
-                field_list, opts, self.get_initial_alias(), False)
+                field_list, opts, join_alias, False)
             # Aggregate references a model or field that requires a join
             self.allow_nulls = True
-
-            col = (join_list[-1], target.column)
+            
+            # here we preform an optimization(the same one as done in add_filter)
+            # if the last table we join to we are joining to the same column as 
+            # the one we are coming from we don't need to do the second join,
+            # which means for an m2m we only join the intermediary table, and not
+            # the final table.  This cuts the joins in half for a query like:
+            # Model.objects.annotate(Count('m2m'))
+            final = len(join_list)
+            penultimate = last.pop()
+            if penultimate == final:
+                penultimate = last.pop()
+            join_alias = join_list[-1]
+            
+            col = target.column
+            
+            while final > 1:
+                join = self.alias_map[join_alias]
+                if col != join[RHS_JOIN_COL]:
+                    break
+                self.unref_alias(join_alias)
+                join_alias = join[LHS_ALIAS]
+                col = join[LHS_JOIN_COL]
+                join_list = join_list[:-1]
+                final -= 1
+                if final == penultimate:
+                    penultimate = last.pop()
+            
+            col = (join_list[-1], col)
             aggregate = aggregate_expr.add_to_query(self, aggregates,
                 col=col,
                 source=target,

tests/regressiontests/aggregation_regress/models.py

@@ -135 +135 @@
 >>> Publisher.objects.annotate(avg_price=Avg('book__price')).aggregate(Max('avg_price'))
 {'avg_price__max': 75.0...}
 
+# Verify that we only join to the intermediary table, instead of having a second
+# join to get to the second table since it isn't necessary.
+>>> Book.objects.annotate(Count('authors')).query.as_sql()[0].count('JOIN')
+1
 """
 }