Pushdown ORDER BY for realtime caggs

Previously ordered queries on realtime caggs would always lead to full
table scan as the query plan would have a sort with the limit on top.
With this patch this gets changed so that the ORDER BY can be pushed
down so the query can benefit from the ordered append optimization and
does not require full table scan.

Since the internal structure is different on PG 14 and 15 this
optimization will only be available on PG 16 and 17.

Fixes #4861

.unreleased/pr_7271

@@ -0,0 +1 @@
+Implements: #7271 Push down ORDER BY in real time continuous aggregate queries

src/cross_module_fn.c

@@ -261,7 +261,7 @@ ts_tsl_loaded(PG_FUNCTION_ARGS)
 }
 
 static void
-preprocess_query_tsl_default_fn_community(Query *parse)
+preprocess_query_tsl_default_fn_community(Query *parse, int *cursor_opts)
 {
 	/* No op in community licensed code */
 }

src/cross_module_fn.h

@@ -152,7 +152,7 @@ typedef struct CrossModuleFunctions
 	PGFunction chunk_unfreeze_chunk;
 	PGFunction recompress_chunk_segmentwise;
 	PGFunction get_compressed_chunk_index_for_recompression;
-	void (*preprocess_query_tsl)(Query *parse);
+	void (*preprocess_query_tsl)(Query *parse, int *cursor_opts);
 } CrossModuleFunctions;
 
 extern TSDLLEXPORT CrossModuleFunctions *ts_cm_functions;

src/guc.c

@@ -10,6 +10,7 @@
 #include <utils/regproc.h>
 #include <utils/varlena.h>
 
+#include "compat/compat.h"
 #include "config.h"
 #include "extension.h"
 #include "guc.h"
@@ -66,6 +67,9 @@ bool ts_guc_enable_qual_propagation = true;
 bool ts_guc_enable_cagg_reorder_groupby = true;
 bool ts_guc_enable_now_constify = true;
 bool ts_guc_enable_foreign_key_propagation = true;
+#if PG16_GE
+TSDLLEXPORT bool ts_guc_enable_cagg_sort_pushdown = true;
+#endif
 TSDLLEXPORT bool ts_guc_enable_cagg_watermark_constify = true;
 TSDLLEXPORT int ts_guc_cagg_max_individual_materializations = 10;
 bool ts_guc_enable_osm_reads = true;
@@ -563,6 +567,19 @@ _guc_init(void)
 							 NULL,
 							 NULL);
 
+#if PG16_GE
+	DefineCustomBoolVariable(MAKE_EXTOPTION("enable_cagg_sort_pushdown"),
+							 "Enable sort pushdown for continuous aggregates",
+							 "Enable pushdown of ORDER BY clause for continuous aggregates",
+							 &ts_guc_enable_cagg_sort_pushdown,
+							 true,
+							 PGC_USERSET,
+							 0,
+							 NULL,
+							 NULL,
+							 NULL);
+#endif
+
 	DefineCustomBoolVariable(MAKE_EXTOPTION("enable_cagg_watermark_constify"),
 							 "Enable cagg watermark constify",
 							 "Enable constifying cagg watermark for real-time caggs",

src/guc.h

@@ -6,6 +6,8 @@
 #pragma once
 
 #include <postgres.h>
+
+#include "compat/compat.h"
 #include "config.h"
 #include "export.h"
 
@@ -27,8 +29,11 @@ extern bool ts_guc_enable_cagg_reorder_groupby;
 extern TSDLLEXPORT int ts_guc_cagg_max_individual_materializations;
 extern bool ts_guc_enable_now_constify;
 extern bool ts_guc_enable_foreign_key_propagation;
-extern TSDLLEXPORT bool ts_guc_enable_cagg_watermark_constify;
 extern bool ts_guc_enable_osm_reads;
+#if PG16_GE
+extern TSDLLEXPORT bool ts_guc_enable_cagg_sort_pushdown;
+#endif
+extern TSDLLEXPORT bool ts_guc_enable_cagg_watermark_constify;
 extern TSDLLEXPORT bool ts_guc_enable_dml_decompression;
 extern TSDLLEXPORT bool ts_guc_enable_dml_decompression_tuple_filtering;
 extern TSDLLEXPORT bool ts_guc_enable_compressed_direct_batch_delete;

src/planner/planner.c

@@ -653,7 +653,7 @@ timescaledb_planner(Query *parse, const char *query_string, int cursor_opts,
 			preprocess_query((Node *) parse, &context);
 
 			if (ts_guc_enable_optimizations)
-				ts_cm_functions->preprocess_query_tsl(parse);
+				ts_cm_functions->preprocess_query_tsl(parse, &cursor_opts);
 		}
 
 		if (prev_planner_hook != NULL)

tsl/src/continuous_aggs/planner.c

@@ -355,3 +355,106 @@ constify_cagg_watermark(Query *parse)
 	if (context.valid_query)
 		replace_watermark_with_const(&context);
 }
+
+#if PG16_GE
+/*
+ * Push down ORDER BY and LIMIT into subqueries of UNION for realtime
+ * continuous aggregates when sorting by time.
+ *
+ * This is only enabled on PG16 and above because the internal structure is different
+ * in previous versions.
+ */
+void
+cagg_sort_pushdown(Query *parse, int *cursor_opts)
+{
+	ListCell *lc;
+
+	/* Nothing to do if we have no valid sort clause */
+	if (list_length(parse->rtable) != 1 || list_length(parse->sortClause) != 1 ||
+		!OidIsValid(linitial_node(SortGroupClause, parse->sortClause)->sortop))
+		return;
+
+	Cache *cache = ts_hypertable_cache_pin();
+
+	foreach (lc, parse->rtable)
+	{
+		RangeTblEntry *rte = lfirst(lc);
+
+		/*
+		 * Realtime cagg view will have 2 rtable entries, one for the materialized data and one for
+		 * the not yet materialized data.
+		 */
+		if (rte->rtekind != RTE_SUBQUERY || rte->relkind != RELKIND_VIEW ||
+			list_length(rte->subquery->rtable) != 2)
+			continue;
+
+		ContinuousAgg *cagg = ts_continuous_agg_find_by_relid(rte->relid);
+
+		/*
+		 * This optimization only applies to realtime caggs.
+		 */
+		if (!cagg || !cagg->data.finalized || cagg->data.materialized_only)
+			continue;
+
+		Hypertable *ht = ts_hypertable_cache_get_entry_by_id(cache, cagg->data.mat_hypertable_id);
+		Dimension const *dim = hyperspace_get_open_dimension(ht->space, 0);
+
+		/* We should only encounter hypertables with an open dimension */
+		if (!dim)
+			continue;
+
+		SortGroupClause *sort = linitial_node(SortGroupClause, parse->sortClause);
+		TargetEntry *tle = get_sortgroupref_tle(sort->tleSortGroupRef, parse->targetList);
+
+		/*
+		 * We only pushdown ORDER BY when it's single column
+		 * ORDER BY on the time column.
+		 */
+		AttrNumber time_col = dim->column_attno;
+		if (!IsA(tle->expr, Var) || castNode(Var, tle->expr)->varattno != time_col)
+			continue;
+
+		RangeTblEntry *mat_rte = linitial_node(RangeTblEntry, rte->subquery->rtable);
+		RangeTblEntry *rt_rte = lsecond_node(RangeTblEntry, rte->subquery->rtable);
+
+		mat_rte->subquery->sortClause = list_copy(parse->sortClause);
+		rt_rte->subquery->sortClause = list_copy(parse->sortClause);
+
+		TargetEntry *mat_tle = list_nth(mat_rte->subquery->targetList, time_col - 1);
+		TargetEntry *rt_tle = list_nth(rt_rte->subquery->targetList, time_col - 1);
+		linitial_node(SortGroupClause, mat_rte->subquery->sortClause)->tleSortGroupRef =
+			mat_tle->ressortgroupref;
+		linitial_node(SortGroupClause, rt_rte->subquery->sortClause)->tleSortGroupRef =
+			rt_tle->ressortgroupref;
+
+		SortGroupClause *cagg_group = linitial(rt_rte->subquery->groupClause);
+		cagg_group = list_nth(rt_rte->subquery->groupClause, rt_tle->ressortgroupref - 1);
+		cagg_group->sortop = sort->sortop;
+		cagg_group->nulls_first = sort->nulls_first;
+
+		Oid placeholder;
+		int16 strategy;
+		get_ordering_op_properties(sort->sortop, &placeholder, &placeholder, &strategy);
+
+		/*
+		 * If this is DESC order and the sortop is the commutator of the cagg_group sortop,
+		 * we can align the sortop of the cagg_group with the sortop of the sort clause, which
+		 * will allow us to have the GroupAggregate node to produce the correct order and avoid
+		 * having to resort.
+		 */
+		if (strategy == BTGreaterStrategyNumber)
+		{
+			rte->subquery->rtable = list_make2(rt_rte, mat_rte);
+		}
+
+		/*
+		 * We have to prevent parallelism when we do this optimization because
+		 * the subplans of the Append have to be processed sequentially.
+		 */
+		*cursor_opts = *cursor_opts & ~CURSOR_OPT_PARALLEL_OK;
+		parse->sortClause = NIL;
+		rte->subquery->sortClause = NIL;
+	}
+	ts_cache_release(cache);
+}
+#endif

tsl/src/continuous_aggs/planner.h

@@ -10,5 +10,6 @@
 #include "planner/planner.h"
 
 void constify_cagg_watermark(Query *parse);
+void cagg_sort_pushdown(Query *parse, int *cursor_opts);
 
 #endif

tsl/src/planner.c

@@ -211,7 +211,7 @@ tsl_set_rel_pathlist(PlannerInfo *root, RelOptInfo *rel, Index rti, RangeTblEntr
  * Run preprocess query optimizations
  */
 void
-tsl_preprocess_query(Query *parse)
+tsl_preprocess_query(Query *parse, int *cursor_opts)
 {
 	Assert(parse != NULL);
 
@@ -220,6 +220,14 @@ tsl_preprocess_query(Query *parse)
 	{
 		constify_cagg_watermark(parse);
 	}
+
+#if PG16_GE
+	/* Push down ORDER BY and LIMIT for realtime cagg (PG16+ only) */
+	if (ts_guc_enable_cagg_sort_pushdown)
+	{
+		cagg_sort_pushdown(parse, cursor_opts);
+	}
+#endif
 }
 
 /*

tsl/src/planner.h

@@ -16,5 +16,5 @@ void tsl_create_upper_paths_hook(PlannerInfo *, UpperRelationKind, RelOptInfo *,
 void tsl_set_rel_pathlist_query(PlannerInfo *, RelOptInfo *, Index, RangeTblEntry *, Hypertable *);
 void tsl_set_rel_pathlist_dml(PlannerInfo *, RelOptInfo *, Index, RangeTblEntry *, Hypertable *);
 void tsl_set_rel_pathlist(PlannerInfo *root, RelOptInfo *rel, Index rti, RangeTblEntry *rte);
-void tsl_preprocess_query(Query *parse);
+void tsl_preprocess_query(Query *parse, int *cursor_opts);
 void tsl_postprocess_plan(PlannedStmt *stmt);