Support for partial aggregations at chunk level

This patch adds support for partial aggregations at the chunk level.
The aggregation is replanned in the create_upper_paths_hook of
PostgreSQL. The AggPath is split up into multiple
AGGSPLIT_INITIAL_SERIAL operations (one on top of each chunk), which
create partials, and one AGGSPLIT_FINAL_DESERIAL operation, which
finalizes the aggregation.

.unreleased/feature_5596

@@ -0,0 +1 @@
+Implements: #5596 Support for partial aggregations at chunk level 

src/guc.c

@@ -83,6 +83,7 @@ TSDLLEXPORT bool ts_guc_enable_decompression_sorted_merge = true;
 bool ts_guc_enable_per_data_node_queries = true;
 bool ts_guc_enable_parameterized_data_node_scan = true;
 bool ts_guc_enable_async_append = true;
+bool ts_guc_enable_chunkwise_aggregation = true;
 TSDLLEXPORT bool ts_guc_enable_compression_indexscan = true;
 TSDLLEXPORT bool ts_guc_enable_bulk_decompression = true;
 TSDLLEXPORT bool ts_guc_enable_skip_scan = true;
@@ -546,6 +547,18 @@ _guc_init(void)
 							 NULL,
 							 NULL);
 
+	DefineCustomBoolVariable("timescaledb.enable_chunkwise_aggregation",
+							 "Enable chunk-wise aggregation",
+							 "Enable the pushdown of aggregations to the"
+							 " chunk level",
+							 &ts_guc_enable_chunkwise_aggregation,
+							 true,
+							 PGC_USERSET,
+							 0,
+							 NULL,
+							 NULL,
+							 NULL);
+
 	DefineCustomBoolVariable("timescaledb.enable_remote_explain",
 							 "Show explain from remote nodes when using VERBOSE flag",
 							 "Enable getting and showing EXPLAIN output from remote nodes",

src/guc.h

@@ -35,6 +35,7 @@ extern TSDLLEXPORT bool ts_guc_enable_per_data_node_queries;
 extern TSDLLEXPORT bool ts_guc_enable_parameterized_data_node_scan;
 extern TSDLLEXPORT bool ts_guc_enable_async_append;
 extern TSDLLEXPORT bool ts_guc_enable_skip_scan;
+extern TSDLLEXPORT bool ts_guc_enable_chunkwise_aggregation;
 extern bool ts_guc_restoring;
 extern int ts_guc_max_open_chunks_per_insert;
 extern int ts_guc_max_cached_chunks_per_hypertable;

src/nodes/chunk_append/planner.c

@@ -402,6 +402,7 @@ ts_chunk_append_get_scan_plan(Plan *plan)
 				return NULL;
 			break;
 		case T_MergeAppend:
+		case T_Agg:
 			return NULL;
 			break;
 		default: