comment about generating grouping paths with extra combine phase

diff --git a/src/backend/optimizer/plan/planner.c b/src/backend/optimizer/plan/planner.c
index 06b172c4db8481a2ab5a563ba6e9e8b843485cfe..70c7d51e2264c9077d57932e1b0ea7cf0d232424 100644 (file)
--- a/src/backend/optimizer/plan/planner.c
+++ b/src/backend/optimizer/plan/planner.c
@@ -4307,6 +4307,25 @@ create_grouping_paths(PlannerInfo *root,
                                }
                        }
                }
+
+               /*
+                * TODO So far we've only constructed simple paths combining partial
+                * and distributed aggregate paths, i.e.
+                *
+                *     Finalize -> RemoteSubplan -> Gather -> Partial
+                *
+                * It may however be more efficient to reduce the amount of data
+                * transferred over the network by generating paths like this:
+                *
+                *     Finalize -> RemoteSubplan -> Combine -> Gather -> Partial
+                *
+                * where Combine deserialized the aggstates, combines them and then
+                * serializes them again. This AggSplit case is not defined yet, but
+                * should not be hard to add.
+                *
+                * We only want to do this for partial paths with RemoteSubplan on
+                * top of them, i.e. when the whole aggregate was not pushed down.
+                */
        }
 
        if (can_hash && !(agg_costs->hasNonPartial || agg_costs->hasNonSerial))
@@ -4370,6 +4389,8 @@ create_grouping_paths(PlannerInfo *root,
                                                                                         dNumGroups));
                        }
                }
+
+               /* TODO Generate the additional paths with extra combine phase here. */
        }
 
        /* Give a helpful error if we failed to find any implementation */