Allow aggregate transition states to be serialized and deserialized.

This is necessary infrastructure for supporting parallel aggregation
for aggregates whose transition type is "internal".  Such values
can't be passed between cooperating processes, because they are
just pointers.

David Rowley, reviewed by Tomas Vondra and by me.

src/backend/optimizer/plan/createplan.c

@@ -1279,6 +1279,7 @@ create_unique_plan(PlannerInfo *root, UniquePath *best_path, int flags)
 								 AGG_HASHED,
 								 false,
 								 true,
+								 false,
 								 numGroupCols,
 								 groupColIdx,
 								 groupOperators,
@@ -1578,6 +1579,7 @@ create_agg_plan(PlannerInfo *root, AggPath *best_path)
 					best_path->aggstrategy,
 					best_path->combineStates,
 					best_path->finalizeAggs,
+					best_path->serialStates,
 					list_length(best_path->groupClause),
 					extract_grouping_cols(best_path->groupClause,
 										  subplan->targetlist),
@@ -1732,6 +1734,7 @@ create_groupingsets_plan(PlannerInfo *root, GroupingSetsPath *best_path)
 										 AGG_SORTED,
 										 false,
 										 true,
+										 false,
 									   list_length((List *) linitial(gsets)),
 										 new_grpColIdx,
 										 extract_grouping_ops(groupClause),
@@ -1768,6 +1771,7 @@ create_groupingsets_plan(PlannerInfo *root, GroupingSetsPath *best_path)
 						(numGroupCols > 0) ? AGG_SORTED : AGG_PLAIN,
 						false,
 						true,
+						false,
 						numGroupCols,
 						top_grpColIdx,
 						extract_grouping_ops(groupClause),
@@ -5636,7 +5640,7 @@ materialize_finished_plan(Plan *subplan)
 Agg *
 make_agg(List *tlist, List *qual,
 		 AggStrategy aggstrategy,
-		 bool combineStates, bool finalizeAggs,
+		 bool combineStates, bool finalizeAggs, bool serialStates,
 		 int numGroupCols, AttrNumber *grpColIdx, Oid *grpOperators,
 		 List *groupingSets, List *chain,
 		 double dNumGroups, Plan *lefttree)
@@ -5651,6 +5655,7 @@ make_agg(List *tlist, List *qual,
 	node->aggstrategy = aggstrategy;
 	node->combineStates = combineStates;
 	node->finalizeAggs = finalizeAggs;
+	node->serialStates = serialStates;
 	node->numCols = numGroupCols;
 	node->grpColIdx = grpColIdx;
 	node->grpOperators = grpOperators;

src/backend/optimizer/plan/planner.c

@@ -3455,7 +3455,8 @@ create_grouping_paths(PlannerInfo *root,
 													&agg_costs,
 													dNumPartialGroups,
 													false,
-													false));
+													false,
+													true));
 					else
 						add_partial_path(grouped_rel, (Path *)
 									create_group_path(root,
@@ -3496,7 +3497,8 @@ create_grouping_paths(PlannerInfo *root,
 											&agg_costs,
 											dNumPartialGroups,
 											false,
-											false));
+											false,
+											true));
 			}
 		}
 	}
@@ -3560,7 +3562,8 @@ create_grouping_paths(PlannerInfo *root,
 											 &agg_costs,
 											 dNumGroups,
 											 false,
-											 true));
+											 true,
+											 false));
 				}
 				else if (parse->groupClause)
 				{
@@ -3626,6 +3629,7 @@ create_grouping_paths(PlannerInfo *root,
 											&agg_costs,
 											dNumGroups,
 											true,
+											true,
 											true));
 			else
 				add_path(grouped_rel, (Path *)
@@ -3668,7 +3672,8 @@ create_grouping_paths(PlannerInfo *root,
 									 &agg_costs,
 									 dNumGroups,
 									 false,
-									 true));
+									 true,
+									 false));
 		}
 
 		/*
@@ -3706,6 +3711,7 @@ create_grouping_paths(PlannerInfo *root,
 											&agg_costs,
 											dNumGroups,
 											true,
+											true,
 											true));
 			}
 		}
@@ -4039,7 +4045,8 @@ create_distinct_paths(PlannerInfo *root,
 								 NULL,
 								 numDistinctRows,
 								 false,
-								 true));
+								 true,
+								 false));
 	}
 
 	/* Give a helpful error if we failed to find any implementation */

src/backend/optimizer/plan/setrefs.c

@@ -2057,10 +2057,10 @@ search_indexed_tlist_for_sortgroupref(Node *node,
  * search_indexed_tlist_for_partial_aggref - find an Aggref in an indexed tlist
  *
  * Aggrefs for partial aggregates have their aggoutputtype adjusted to set it
- * to the aggregate state's type. This means that a standard equal() comparison
- * won't match when comparing an Aggref which is in partial mode with an Aggref
- * which is not. Here we manually compare all of the fields apart from
- * aggoutputtype.
+ * to the aggregate state's type, or serialization type. This means that a
+ * standard equal() comparison won't match when comparing an Aggref which is
+ * in partial mode with an Aggref which is not. Here we manually compare all of
+ * the fields apart from aggoutputtype.
  */
 static Var *
 search_indexed_tlist_for_partial_aggref(Aggref *aggref, indexed_tlist *itlist,

src/backend/optimizer/prep/prepunion.c

@@ -861,7 +861,8 @@ make_union_unique(SetOperationStmt *op, Path *path, List *tlist,
 										NULL,
 										dNumGroups,
 										false,
-										true);
+										true,
+										false);
 	}
 	else
 	{

src/backend/optimizer/util/clauses.c

@@ -464,11 +464,15 @@ aggregates_allow_partial_walker(Node *node, partial_agg_context *context)
 		}
 
 		/*
-		 * If we find any aggs with an internal transtype then we must ensure
-		 * that pointers to aggregate states are not passed to other processes;
-		 * therefore, we set the maximum allowed type to PAT_INTERNAL_ONLY.
+		 * If we find any aggs with an internal transtype then we must check
+		 * that these have a serialization type, serialization func and
+		 * deserialization func; otherwise, we set the maximum allowed type to
+		 * PAT_INTERNAL_ONLY.
 		 */
-		if (aggform->aggtranstype == INTERNALOID)
+		if (aggform->aggtranstype == INTERNALOID &&
+			(!OidIsValid(aggform->aggserialtype) ||
+			 !OidIsValid(aggform->aggserialfn) ||
+			 !OidIsValid(aggform->aggdeserialfn)))
 			context->allowedtype = PAT_INTERNAL_ONLY;
 
 		ReleaseSysCache(aggTuple);

src/backend/optimizer/util/pathnode.c

@@ -2433,7 +2433,8 @@ create_agg_path(PlannerInfo *root,
 				const AggClauseCosts *aggcosts,
 				double numGroups,
 				bool combineStates,
-				bool finalizeAggs)
+				bool finalizeAggs,
+				bool serialStates)
 {
 	AggPath    *pathnode = makeNode(AggPath);
 
@@ -2458,6 +2459,7 @@ create_agg_path(PlannerInfo *root,
 	pathnode->qual = qual;
 	pathnode->finalizeAggs = finalizeAggs;
 	pathnode->combineStates = combineStates;
+	pathnode->serialStates = serialStates;
 
 	cost_agg(&pathnode->path, root,
 			 aggstrategy, aggcosts,

src/backend/optimizer/util/tlist.c

@@ -756,8 +756,8 @@ apply_pathtarget_labeling_to_tlist(List *tlist, PathTarget *target)
  * apply_partialaggref_adjustment
  *	  Convert PathTarget to be suitable for a partial aggregate node. We simply
  *	  adjust any Aggref nodes found in the target and set the aggoutputtype to
- *	  the aggtranstype. This allows exprType() to return the actual type that
- *	  will be produced.
+ *	  the aggtranstype or aggserialtype. This allows exprType() to return the
+ *	  actual type that will be produced.
  *
  * Note: We expect 'target' to be a flat target list and not have Aggrefs burried
  * within other expressions.
@@ -785,7 +785,12 @@ apply_partialaggref_adjustment(PathTarget *target)
 			aggform = (Form_pg_aggregate) GETSTRUCT(aggTuple);
 
 			newaggref = (Aggref *) copyObject(aggref);
-			newaggref->aggoutputtype = aggform->aggtranstype;
+
+			/* use the serialization type, if one exists */
+			if (OidIsValid(aggform->aggserialtype))
+				newaggref->aggoutputtype = aggform->aggserialtype;
+			else
+				newaggref->aggoutputtype = aggform->aggtranstype;
 
 			lfirst(lc) = newaggref;