Avoid scan on hypertable relation (#9722)

It's always empty, so this scan is useless and looks misleading. The
common case when all chunks are excluded was already handled, but the
cases of SELECT FROM ONLY hypertable or an empty hypertable were not.

Author: akuzm

src/planner/planner.c

@@ -1470,8 +1470,8 @@ timescaledb_set_rel_pathlist(PlannerInfo *root, RelOptInfo *rel, Index rti, Rang
 			{
 				ts_planner_constraint_cleanup(root, rel);
 			}
-
 			break;
+
 		case TS_REL_CHUNK_STANDALONE:
 		case TS_REL_CHUNK_CHILD:
 			/* Check for UPDATE/DELETE/MERGE (DML) on compressed chunks */
@@ -1482,10 +1482,14 @@ timescaledb_set_rel_pathlist(PlannerInfo *root, RelOptInfo *rel, Index rti, Rang
 				{
 					ts_cm_functions->set_rel_pathlist_dml(root, rel, rti, rte, ht);
 				}
-				break;
 			}
-			TS_FALLTHROUGH;
-		default:
+			else
+			{
+				apply_optimizations(root, reltype, rel, rte, ht);
+			}
+			break;
+
+		case TS_REL_HYPERTABLE:
 			/*
 			 * Set the indexlist for a hypertable parent to NIL since we
 			 * should not try to do any index scans on hypertable parents,
@@ -1497,10 +1501,26 @@ timescaledb_set_rel_pathlist(PlannerInfo *root, RelOptInfo *rel, Index rti, Rang
 			 * This has to be after the hypertable is expanded, since the
 			 * indexlist is used during hypertable expansion.
 			 */
-			if (reltype == TS_REL_HYPERTABLE)
+
+			rel->indexlist = NIL;
+
+			if (!rte->inh)
+			{
+				/*
+				 * This happens with SELECT FROM ONLY hypertable or with an
+				 * empty hypertable. Mark it as dummy, otherwise we'll get a
+				 * scan on hypertable relation itself. It's always empty, so
+				 * this scan is useless and looks misleading.
+				 */
+				mark_dummy_rel(rel);
+			}
+			else
 			{
-				rel->indexlist = NIL;
+				apply_optimizations(root, reltype, rel, rte, ht);
 			}
+			break;
+
+		default:
 			apply_optimizations(root, reltype, rel, rte, ht);
 			break;
 	}

test/expected/ddl.out

@@ -259,7 +259,8 @@ SELECT * FROM ONLY PUBLIC."Hypertable_1";
 
 EXPLAIN (buffers off, costs off) SELECT * FROM ONLY PUBLIC."Hypertable_1";
 --- QUERY PLAN ---
- Seq Scan on "Hypertable_1"
+ Result
+   One-Time Filter: false
 
 SELECT * FROM test.show_columns('PUBLIC."Hypertable_1"');
   Column   |  Type   | NotNull 

test/expected/dml_system_columns.out

@@ -434,35 +434,37 @@ VACUUM FREEZE ANALYZE ht_zero_chunks;
 --- QUERY PLAN ---
  Custom Scan (ModifyHypertable) (actual rows=0.00 loops=1)
    ->  Update on public.ht_zero_chunks (actual rows=0.00 loops=1)
-         ->  Seq Scan on public.ht_zero_chunks (actual rows=0.00 loops=1)
+         ->  Result (actual rows=0.00 loops=1)
                Output: 1, ctid
-               Filter: (ht_zero_chunks."time" = 'Wed Jan 15 00:00:00 2020 PST'::timestamp with time zone)
+               One-Time Filter: false
 
 :PREFIX DELETE FROM ht_zero_chunks WHERE time = '2020-01-15';
 --- QUERY PLAN ---
  Custom Scan (ModifyHypertable) (actual rows=0.00 loops=1)
    ->  Delete on public.ht_zero_chunks (actual rows=0.00 loops=1)
-         ->  Seq Scan on public.ht_zero_chunks (actual rows=0.00 loops=1)
+         ->  Result (actual rows=0.00 loops=1)
                Output: ctid
-               Filter: (ht_zero_chunks."time" = 'Wed Jan 15 00:00:00 2020 PST'::timestamp with time zone)
+               One-Time Filter: false
 
 :PREFIX UPDATE ht_zero_chunks SET v = 1 RETURNING ctid, xmin, tableoid::regclass, *;
 --- QUERY PLAN ---
  Custom Scan (ModifyHypertable) (actual rows=0.00 loops=1)
    Output: ctid, xmin, ((tableoid)::regclass), "time", v
    ->  Update on public.ht_zero_chunks (actual rows=0.00 loops=1)
          Output: ctid, xmin, (tableoid)::regclass, "time", v
-         ->  Seq Scan on public.ht_zero_chunks (actual rows=0.00 loops=1)
+         ->  Result (actual rows=0.00 loops=1)
                Output: 1, ctid
+               One-Time Filter: false
 
 :PREFIX DELETE FROM ht_zero_chunks RETURNING ctid, xmin, tableoid::regclass, *;
 --- QUERY PLAN ---
  Custom Scan (ModifyHypertable) (actual rows=0.00 loops=1)
    Output: ctid, xmin, ((tableoid)::regclass), "time", v
    ->  Delete on public.ht_zero_chunks (actual rows=0.00 loops=1)
          Output: ctid, xmin, (tableoid)::regclass, "time", v
-         ->  Seq Scan on public.ht_zero_chunks (actual rows=0.00 loops=1)
+         ->  Result (actual rows=0.00 loops=1)
                Output: ctid
+               One-Time Filter: false
 
 DROP TABLE ht_zero_chunks;
 -- UPDATE/DELETE on a hypertable that becomes a dummy rel due to

test/expected/plan_expand_hypertable-15.out

@@ -3236,35 +3236,21 @@ EXPLAIN (buffers off, costs off) SELECT * FROM f_t1_2(10);
  Subquery Scan on f_t1_2
    ->  Unique
          ->  Sort
-               Sort Key: j.a
-               ->  Nested Loop
-                     ->  Seq Scan on t1 j
-                     ->  Unique
-                           ->  Index Scan using t1_b_idx on t1
-                                 Index Cond: (b = 10)
-                                 Filter: (a = j.a)
+               Sort Key: a
+               ->  Result
+                     One-Time Filter: false
 
 EXPLAIN (buffers off, costs off) SELECT * FROM f_t1_2(10) sc, f_t2(sc.a, 10);
 --- QUERY PLAN ---
  Nested Loop
    ->  Unique
          ->  Sort
                Sort Key: j.a
-               ->  Nested Loop
-                     ->  Seq Scan on t1 j
-                     ->  Unique
-                           ->  Index Scan using t1_b_idx on t1
-                                 Index Cond: (b = 10)
-                                 Filter: (a = j.a)
+               ->  Result
+                     One-Time Filter: false
    ->  Unique
-         ->  Nested Loop
-               ->  Unique
-                     ->  Index Scan using t1_b_idx on t1 t1_1
-                           Index Cond: (b = 10)
-                           Filter: (a = t1.a)
-               ->  Index Scan using t2_b_idx on t2 j_1
-                     Index Cond: (b = 10)
-                     Filter: (a = t1.a)
+         ->  Result
+               One-Time Filter: false
 
 CREATE TABLE metrics_int1(time int, device text, value float) WITH (tsdb.hypertable,tsdb.partition_column='time',tsdb.chunk_interval=1);
 INSERT INTO metrics_int1 SELECT i, i::text, i FROM generate_series(3,7) i;

test/expected/plan_expand_hypertable-16.out

@@ -3234,35 +3234,21 @@ EXPLAIN (buffers off, costs off) SELECT * FROM f_t1_2(10);
  Subquery Scan on f_t1_2
    ->  Unique
          ->  Sort
-               Sort Key: j.a
-               ->  Nested Loop
-                     ->  Seq Scan on t1 j
-                     ->  Limit
-                           ->  Index Scan using t1_b_idx on t1
-                                 Index Cond: (b = 10)
-                                 Filter: (a = j.a)
+               Sort Key: a
+               ->  Result
+                     One-Time Filter: false
 
 EXPLAIN (buffers off, costs off) SELECT * FROM f_t1_2(10) sc, f_t2(sc.a, 10);
 --- QUERY PLAN ---
  Nested Loop
    ->  Unique
          ->  Sort
                Sort Key: j.a
-               ->  Nested Loop
-                     ->  Seq Scan on t1 j
-                     ->  Limit
-                           ->  Index Scan using t1_b_idx on t1
-                                 Index Cond: (b = 10)
-                                 Filter: (a = j.a)
+               ->  Result
+                     One-Time Filter: false
    ->  Limit
-         ->  Nested Loop
-               ->  Limit
-                     ->  Index Scan using t1_b_idx on t1 t1_1
-                           Index Cond: (b = 10)
-                           Filter: (a = t1.a)
-               ->  Index Scan using t2_b_idx on t2 j_1
-                     Index Cond: (b = 10)
-                     Filter: (a = t1.a)
+         ->  Result
+               One-Time Filter: false
 
 CREATE TABLE metrics_int1(time int, device text, value float) WITH (tsdb.hypertable,tsdb.partition_column='time',tsdb.chunk_interval=1);
 INSERT INTO metrics_int1 SELECT i, i::text, i FROM generate_series(3,7) i;

test/expected/plan_expand_hypertable-17.out

@@ -3234,35 +3234,21 @@ EXPLAIN (buffers off, costs off) SELECT * FROM f_t1_2(10);
  Subquery Scan on f_t1_2
    ->  Unique
          ->  Sort
-               Sort Key: j.a
-               ->  Nested Loop
-                     ->  Seq Scan on t1 j
-                     ->  Limit
-                           ->  Index Scan using t1_b_idx on t1
-                                 Index Cond: (b = 10)
-                                 Filter: (a = j.a)
+               Sort Key: a
+               ->  Result
+                     One-Time Filter: false
 
 EXPLAIN (buffers off, costs off) SELECT * FROM f_t1_2(10) sc, f_t2(sc.a, 10);
 --- QUERY PLAN ---
  Nested Loop
    ->  Unique
          ->  Sort
                Sort Key: j.a
-               ->  Nested Loop
-                     ->  Seq Scan on t1 j
-                     ->  Limit
-                           ->  Index Scan using t1_b_idx on t1
-                                 Index Cond: (b = 10)
-                                 Filter: (a = j.a)
+               ->  Result
+                     One-Time Filter: false
    ->  Limit
-         ->  Nested Loop
-               ->  Limit
-                     ->  Index Scan using t1_b_idx on t1 t1_1
-                           Index Cond: (b = 10)
-                           Filter: (a = t1.a)
-               ->  Index Scan using t2_b_idx on t2 j_1
-                     Index Cond: (b = 10)
-                     Filter: (a = t1.a)
+         ->  Result
+               One-Time Filter: false
 
 CREATE TABLE metrics_int1(time int, device text, value float) WITH (tsdb.hypertable,tsdb.partition_column='time',tsdb.chunk_interval=1);
 INSERT INTO metrics_int1 SELECT i, i::text, i FROM generate_series(3,7) i;

test/expected/plan_expand_hypertable-18.out

@@ -3234,35 +3234,21 @@ EXPLAIN (buffers off, costs off) SELECT * FROM f_t1_2(10);
  Subquery Scan on f_t1_2
    ->  Unique
          ->  Sort
-               Sort Key: j.a
-               ->  Nested Loop
-                     ->  Seq Scan on t1 j
-                     ->  Limit
-                           ->  Index Scan using t1_b_idx on t1
-                                 Index Cond: (b = 10)
-                                 Filter: (a = j.a)
+               Sort Key: a
+               ->  Result
+                     One-Time Filter: false
 
 EXPLAIN (buffers off, costs off) SELECT * FROM f_t1_2(10) sc, f_t2(sc.a, 10);
 --- QUERY PLAN ---
  Nested Loop
    ->  Unique
          ->  Sort
                Sort Key: j.a
-               ->  Nested Loop
-                     ->  Seq Scan on t1 j
-                     ->  Limit
-                           ->  Index Scan using t1_b_idx on t1
-                                 Index Cond: (b = 10)
-                                 Filter: (a = j.a)
+               ->  Result
+                     One-Time Filter: false
    ->  Limit
-         ->  Nested Loop
-               ->  Limit
-                     ->  Index Scan using t1_b_idx on t1 t1_1
-                           Index Cond: (b = 10)
-                           Filter: (a = t1.a)
-               ->  Index Scan using t2_b_idx on t2 j_1
-                     Index Cond: (b = 10)
-                     Filter: (a = t1.a)
+         ->  Result
+               One-Time Filter: false
 
 CREATE TABLE metrics_int1(time int, device text, value float) WITH (tsdb.hypertable,tsdb.partition_column='time',tsdb.chunk_interval=1);
 INSERT INTO metrics_int1 SELECT i, i::text, i FROM generate_series(3,7) i;