From: Tomas Vondra <tomas@2ndquadrant.com>
Date: Sat, 14 Jan 2017 19:51:55 +0000 (+0100)
Subject: comment about generating grouping paths with extra combine phase
X-Git-Tag: XL_10_R1BETA1~463
X-Git-Url: http://git.postgresql.org/gitweb/static/gitweb.js?a=commitdiff_plain;h=4ab1534b007ac78d3a9d16728a00a7eb3081a1a0;p=postgres-xl.git

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 06b172c4db..70c7d51e22 100644
--- 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 */