Rework HashAgg GUCs.

Eliminate enable_groupingsets_hash_disk, which was primarily useful
for testing grouping sets that use HashAgg and spill. Instead, hack
the table stats to convince the planner to choose hashed aggregation
for grouping sets that will spill to disk. Suggested by Melanie
Plageman.

Rename enable_hashagg_disk to hashagg_avoid_disk_plan, and invert the
meaning of on/off. The new name indicates more strongly that it only
affects the planner. Also, the word "avoid" is less definite, which
should avoid surprises when HashAgg still needs to use the
disk. Change suggested by Justin Pryzby, though I chose a different
GUC name.

Discussion: https://postgr.es/m/CAAKRu_aisiENMsPM2gC4oUY1hHG3yrCwY-fXUg22C6_MJUwQdA%40mail.gmail.com
Discussion: https://postgr.es/m/20200610021544.GA14879@telsasoft.com
Backpatch-through: 13

doc/src/sgml/config.sgml

@@ -4546,23 +4546,6 @@ ANY <replaceable class="parameter">num_sync</replaceable> ( <replaceable class="
       </listitem>
      </varlistentry>
 
-     <varlistentry id="guc-enable-groupingsets-hash-disk" xreflabel="enable_groupingsets_hash_disk">
-      <term><varname>enable_groupingsets_hash_disk</varname> (<type>boolean</type>)
-      <indexterm>
-       <primary><varname>enable_groupingsets_hash_disk</varname> configuration parameter</primary>
-      </indexterm>
-      </term>
-      <listitem>
-       <para>
-        Enables or disables the query planner's use of hashed aggregation plan
-        types for grouping sets when the total size of the hash tables is
-        expected to exceed <varname>work_mem</varname>.  See <xref
-        linkend="queries-grouping-sets"/>.  The default is
-        <literal>off</literal>.
-       </para>
-      </listitem>
-     </varlistentry>
-
      <varlistentry id="guc-enable-hashagg" xreflabel="enable_hashagg">
       <term><varname>enable_hashagg</varname> (<type>boolean</type>)
       <indexterm>
@@ -4577,17 +4560,19 @@ ANY <replaceable class="parameter">num_sync</replaceable> ( <replaceable class="
       </listitem>
      </varlistentry>
 
-     <varlistentry id="guc-enable-hashagg-disk" xreflabel="enable_hashagg_disk">
-      <term><varname>enable_hashagg_disk</varname> (<type>boolean</type>)
+     <varlistentry id="guc-hashagg-avoid-disk-plan" xreflabel="hashagg_avoid_disk_plan">
+      <term><varname>hashagg_avoid_disk_plan</varname> (<type>boolean</type>)
       <indexterm>
-       <primary><varname>enable_hashagg_disk</varname> configuration parameter</primary>
+       <primary><varname>hashagg_avoid_disk_plan</varname> configuration parameter</primary>
       </indexterm>
       </term>
       <listitem>
        <para>
-        Enables or disables the query planner's use of hashed aggregation plan
-        types when the memory usage is expected to exceed
-        <varname>work_mem</varname>.  The default is <literal>on</literal>.
+        If set to <literal>on</literal>, causes the planner to avoid choosing
+        hashed aggregation plans that are expected to use the disk. If hashed
+        aggregation is chosen, it may still require the use of disk at
+        execution time, even if this parameter is enabled. The default is
+        <literal>off</literal>.
        </para>
       </listitem>
      </varlistentry>

src/backend/optimizer/path/costsize.c

@@ -130,8 +130,7 @@ bool		enable_tidscan = true;
 bool		enable_sort = true;
 bool		enable_incrementalsort = true;
 bool		enable_hashagg = true;
-bool		enable_hashagg_disk = true;
-bool		enable_groupingsets_hash_disk = false;
+bool		hashagg_avoid_disk_plan = true;
 bool		enable_nestloop = true;
 bool		enable_material = true;
 bool		enable_mergejoin = true;

src/backend/optimizer/plan/planner.c

@@ -4256,12 +4256,11 @@ consider_groupingsets_paths(PlannerInfo *root,
 											  dNumGroups - exclude_groups);
 
 		/*
-		 * If we have sortable columns to work with (gd->rollups is non-empty)
-		 * and enable_groupingsets_hash_disk is disabled, don't generate
-		 * hash-based paths that will exceed work_mem.
+		 * gd->rollups is empty if we have only unsortable columns to work
+		 * with.  Override work_mem in that case; otherwise, we'll rely on the
+		 * sorted-input case to generate usable mixed paths.
 		 */
-		if (!enable_groupingsets_hash_disk &&
-			hashsize > work_mem * 1024L && gd->rollups)
+		if (hashsize > work_mem * 1024L && gd->rollups)
 			return;				/* nope, won't fit */
 
 		/*
@@ -4868,7 +4867,7 @@ create_distinct_paths(PlannerInfo *root,
 	{
 		Size		hashentrysize = hash_agg_entry_size(0, cheapest_input_path->pathtarget->width, 0);
 
-		allow_hash = enable_hashagg_disk ||
+		allow_hash = !hashagg_avoid_disk_plan ||
 			(hashentrysize * numDistinctRows <= work_mem * 1024L);
 	}
 
@@ -6773,7 +6772,7 @@ add_paths_to_grouping_rel(PlannerInfo *root, RelOptInfo *input_rel,
 			 * were unable to sort above, then we'd better generate a Path, so
 			 * that we at least have one.
 			 */
-			if (enable_hashagg_disk ||
+			if (!hashagg_avoid_disk_plan ||
 				hashaggtablesize < work_mem * 1024L ||
 				grouped_rel->pathlist == NIL)
 			{
@@ -6807,7 +6806,7 @@ add_paths_to_grouping_rel(PlannerInfo *root, RelOptInfo *input_rel,
 														  agg_final_costs,
 														  dNumGroups);
 
-			if (enable_hashagg_disk ||
+			if (!hashagg_avoid_disk_plan ||
 				hashaggtablesize < work_mem * 1024L)
 				add_path(grouped_rel, (Path *)
 						 create_agg_path(root,
@@ -7188,7 +7187,7 @@ create_partial_grouping_paths(PlannerInfo *root,
 		 * Tentatively produce a partial HashAgg Path, depending on if it
 		 * looks as if the hash table will fit in work_mem.
 		 */
-		if ((enable_hashagg_disk || hashaggtablesize < work_mem * 1024L) &&
+		if ((!hashagg_avoid_disk_plan || hashaggtablesize < work_mem * 1024L) &&
 			cheapest_total_path != NULL)
 		{
 			add_path(partially_grouped_rel, (Path *)
@@ -7215,7 +7214,8 @@ create_partial_grouping_paths(PlannerInfo *root,
 									   dNumPartialPartialGroups);
 
 		/* Do the same for partial paths. */
-		if ((enable_hashagg_disk || hashaggtablesize < work_mem * 1024L) &&
+		if ((!hashagg_avoid_disk_plan ||
+			 hashaggtablesize < work_mem * 1024L) &&
 			cheapest_partial_path != NULL)
 		{
 			add_partial_path(partially_grouped_rel, (Path *)

src/backend/utils/misc/guc.c

@@ -1002,22 +1002,12 @@ static struct config_bool ConfigureNamesBool[] =
 		NULL, NULL, NULL
 	},
 	{
-		{"enable_hashagg_disk", PGC_USERSET, QUERY_TUNING_METHOD,
-			gettext_noop("Enables the planner's use of hashed aggregation plans that are expected to exceed work_mem."),
+		{"hashagg_avoid_disk_plan", PGC_USERSET, QUERY_TUNING_METHOD,
+			gettext_noop("Causes the planner to avoid hashed aggregation plans that are expected to use the disk."),
 			NULL,
 			GUC_EXPLAIN
 		},
-		&enable_hashagg_disk,
-		true,
-		NULL, NULL, NULL
-	},
-	{
-		{"enable_groupingsets_hash_disk", PGC_USERSET, QUERY_TUNING_METHOD,
-			gettext_noop("Enables the planner's use of hashed aggregation plans for groupingsets when the total size of the hash tables is expected to exceed work_mem."),
-			NULL,
-			GUC_EXPLAIN
-		},
-		&enable_groupingsets_hash_disk,
+		&hashagg_avoid_disk_plan,
 		false,
 		NULL, NULL, NULL
 	},

src/include/optimizer/cost.h

@@ -55,8 +55,7 @@ extern PGDLLIMPORT bool enable_tidscan;
 extern PGDLLIMPORT bool enable_sort;
 extern PGDLLIMPORT bool enable_incrementalsort;
 extern PGDLLIMPORT bool enable_hashagg;
-extern PGDLLIMPORT bool enable_hashagg_disk;
-extern PGDLLIMPORT bool enable_groupingsets_hash_disk;
+extern PGDLLIMPORT bool hashagg_avoid_disk_plan;
 extern PGDLLIMPORT bool enable_nestloop;
 extern PGDLLIMPORT bool enable_material;
 extern PGDLLIMPORT bool enable_mergejoin;

src/test/regress/expected/aggregates.out

@@ -2449,26 +2449,30 @@ set enable_sort to default;
 -- aggregation. Force spilling in both cases by setting work_mem low.
 --
 set work_mem='64kB';
+create table agg_data_2k as
+select g from generate_series(0, 1999) g;
+analyze agg_data_2k;
+create table agg_data_20k as
+select g from generate_series(0, 19999) g;
+analyze agg_data_20k;
 -- Produce results with sorting.
 set enable_hashagg = false;
 set jit_above_cost = 0;
 explain (costs off)
 select g%10000 as c1, sum(g::numeric) as c2, count(*) as c3
-  from generate_series(0, 19999) g
-  group by g%10000;
-                   QUERY PLAN                   
-------------------------------------------------
+  from agg_data_20k group by g%10000;
+              QUERY PLAN              
+--------------------------------------
  GroupAggregate
    Group Key: ((g % 10000))
    ->  Sort
          Sort Key: ((g % 10000))
-         ->  Function Scan on generate_series g
+         ->  Seq Scan on agg_data_20k
 (5 rows)
 
 create table agg_group_1 as
 select g%10000 as c1, sum(g::numeric) as c2, count(*) as c3
-  from generate_series(0, 19999) g
-  group by g%10000;
+  from agg_data_20k group by g%10000;
 create table agg_group_2 as
 select * from
   (values (100), (300), (500)) as r(a),
@@ -2476,37 +2480,33 @@ select * from
     select (g/2)::numeric as c1,
            array_agg(g::numeric) as c2,
 	   count(*) as c3
-    from generate_series(0, 1999) g
+    from agg_data_2k
     where g < r.a
     group by g/2) as s;
 set jit_above_cost to default;
 create table agg_group_3 as
 select (g/2)::numeric as c1, sum(7::int4) as c2, count(*) as c3
-  from generate_series(0, 1999) g
-  group by g/2;
+  from agg_data_2k group by g/2;
 create table agg_group_4 as
 select (g/2)::numeric as c1, array_agg(g::numeric) as c2, count(*) as c3
-  from generate_series(0, 1999) g
-  group by g/2;
+  from agg_data_2k group by g/2;
 -- Produce results with hash aggregation
 set enable_hashagg = true;
 set enable_sort = false;
 set jit_above_cost = 0;
 explain (costs off)
 select g%10000 as c1, sum(g::numeric) as c2, count(*) as c3
-  from generate_series(0, 19999) g
-  group by g%10000;
-                QUERY PLAN                
-------------------------------------------
+  from agg_data_20k group by g%10000;
+           QUERY PLAN           
+--------------------------------
  HashAggregate
    Group Key: (g % 10000)
-   ->  Function Scan on generate_series g
+   ->  Seq Scan on agg_data_20k
 (3 rows)
 
 create table agg_hash_1 as
 select g%10000 as c1, sum(g::numeric) as c2, count(*) as c3
-  from generate_series(0, 19999) g
-  group by g%10000;
+  from agg_data_20k group by g%10000;
 create table agg_hash_2 as
 select * from
   (values (100), (300), (500)) as r(a),
@@ -2514,18 +2514,16 @@ select * from
     select (g/2)::numeric as c1,
            array_agg(g::numeric) as c2,
 	   count(*) as c3
-    from generate_series(0, 1999) g
+    from agg_data_2k
     where g < r.a
     group by g/2) as s;
 set jit_above_cost to default;
 create table agg_hash_3 as
 select (g/2)::numeric as c1, sum(7::int4) as c2, count(*) as c3
-  from generate_series(0, 1999) g
-  group by g/2;
+  from agg_data_2k group by g/2;
 create table agg_hash_4 as
 select (g/2)::numeric as c1, array_agg(g::numeric) as c2, count(*) as c3
-  from generate_series(0, 1999) g
-  group by g/2;
+  from agg_data_2k group by g/2;
 set enable_sort = true;
 set work_mem to default;
 -- Compare group aggregation results to hash aggregation results

src/test/regress/expected/groupingsets.out

@@ -1636,68 +1636,65 @@ select v||'a', case when grouping(v||'a') = 1 then 1 else 0 end, count(*)
 --
 -- Compare results between plans using sorting and plans using hash
 -- aggregation. Force spilling in both cases by setting work_mem low
+-- and altering the statistics.
 --
---
+create table gs_data_1 as
+select g%1000 as g1000, g%100 as g100, g%10 as g10, g
+   from generate_series(0,1999) g;
+analyze gs_data_1;
+alter table gs_data_1 set (autovacuum_enabled = 'false');
+update pg_class set reltuples = 10 where relname='gs_data_1';
 SET work_mem='64kB';
 -- Produce results with sorting.
 set enable_hashagg = false;
 set jit_above_cost = 0;
 explain (costs off)
-explain (costs off)
-select g100, g10, sum(g::numeric), count(*), max(g::text) from
-  (select g%1000 as g1000, g%100 as g100, g%10 as g10, g
-   from generate_series(0,1999) g) s
-group by cube (g1000, g100,g10);
-                          QUERY PLAN                           
+select g100, g10, sum(g::numeric), count(*), max(g::text)
+from gs_data_1 group by cube (g1000, g100,g10);
+             QUERY PLAN             
+------------------------------------
  GroupAggregate
- GroupAggregate
-   Group Key: ((g.g % 1000)), ((g.g % 100)), ((g.g % 10))
-   Group Key: ((g.g % 1000)), ((g.g % 100))
+   Group Key: g1000, g100, g10
+   Group Key: g1000, g100
+   Group Key: g1000
    Group Key: ()
-   Group Key: ()
-   Sort Key: ((g.g % 100)), ((g.g % 10))
-     Group Key: ((g.g % 100)), ((g.g % 10))
-     Group Key: ((g.g % 100))
-   Sort Key: ((g.g % 10)), ((g.g % 1000))
-     Group Key: ((g.g % 10)), ((g.g % 1000))
+   Sort Key: g100, g10
+     Group Key: g100, g10
+     Group Key: g100
+   Sort Key: g10, g1000
+     Group Key: g10, g1000
+     Group Key: g10
    ->  Sort
-   ->  Sort
-         Sort Key: ((g.g % 1000)), ((g.g % 100)), ((g.g % 10))
+         Sort Key: g1000, g100, g10
+         ->  Seq Scan on gs_data_1
 (14 rows)
 
 create table gs_group_1 as
-create table gs_group_1 as
-select g100, g10, sum(g::numeric), count(*), max(g::text) from
-  (select g%1000 as g1000, g%100 as g100, g%10 as g10, g
-   from generate_series(0,1999) g) s
+select g100, g10, sum(g::numeric), count(*), max(g::text)
+from gs_data_1 group by cube (g1000, g100,g10);
 -- Produce results with hash aggregation.
 set enable_hashagg = true;
 set enable_sort = false;
 explain (costs off)
-explain (costs off)
-select g100, g10, sum(g::numeric), count(*), max(g::text) from
-  (select g%1000 as g1000, g%100 as g100, g%10 as g10, g
-   from generate_series(0,1999) g) s
-group by cube (g1000, g100,g10);
-                    QUERY PLAN                     
+select g100, g10, sum(g::numeric), count(*), max(g::text)
+from gs_data_1 group by cube (g1000, g100,g10);
+          QUERY PLAN          
+------------------------------
  MixedAggregate
- MixedAggregate
-   Hash Key: (g.g % 1000), (g.g % 100), (g.g % 10)
-   Hash Key: (g.g % 1000), (g.g % 100)
-   Hash Key: (g.g % 1000)
-   Hash Key: (g.g % 100), (g.g % 10)
-   Hash Key: (g.g % 100)
-   Hash Key: (g.g % 10), (g.g % 1000)
+   Hash Key: g1000, g100, g10
+   Hash Key: g1000, g100
+   Hash Key: g1000
+   Hash Key: g100, g10
+   Hash Key: g100
+   Hash Key: g10, g1000
+   Hash Key: g10
    Group Key: ()
-   Group Key: ()
+   ->  Seq Scan on gs_data_1
 (10 rows)
 
 create table gs_hash_1 as
-create table gs_hash_1 as
-select g100, g10, sum(g::numeric), count(*), max(g::text) from
-  (select g%1000 as g1000, g%100 as g100, g%10 as g10, g
-   from generate_series(0,1999) g) s
+select g100, g10, sum(g::numeric), count(*), max(g::text)
+from gs_data_1 group by cube (g1000, g100,g10);
 set enable_sort = true;
 set work_mem to default;
 -- Compare results
@@ -1710,5 +1707,4 @@ set work_mem to default;
 
 drop table gs_group_1;
 drop table gs_hash_1;
-SET enable_groupingsets_hash_disk TO DEFAULT;
 -- end

src/test/regress/expected/sysviews.out

@@ -74,9 +74,7 @@ select name, setting from pg_settings where name like 'enable%';
 --------------------------------+---------
  enable_bitmapscan              | on
  enable_gathermerge             | on
- enable_groupingsets_hash_disk  | off
  enable_hashagg                 | on
- enable_hashagg_disk            | on
  enable_hashjoin                | on
  enable_incrementalsort         | on
  enable_indexonlyscan           | on
@@ -92,7 +90,7 @@ select name, setting from pg_settings where name like 'enable%';
  enable_seqscan                 | on
  enable_sort                    | on
  enable_tidscan                 | on
-(20 rows)
+(18 rows)
 
 -- Test that the pg_timezone_names and pg_timezone_abbrevs views are
 -- more-or-less working.  We can't test their contents in any great detail

src/test/regress/sql/aggregates.sql

@@ -1059,6 +1059,14 @@ set enable_sort to default;
 
 set work_mem='64kB';
 
+create table agg_data_2k as
+select g from generate_series(0, 1999) g;
+analyze agg_data_2k;
+
+create table agg_data_20k as
+select g from generate_series(0, 19999) g;
+analyze agg_data_20k;
+
 -- Produce results with sorting.
 
 set enable_hashagg = false;
@@ -1067,13 +1075,11 @@ set jit_above_cost = 0;
 
 explain (costs off)
 select g%10000 as c1, sum(g::numeric) as c2, count(*) as c3
-  from generate_series(0, 19999) g
-  group by g%10000;
+  from agg_data_20k group by g%10000;
 
 create table agg_group_1 as
 select g%10000 as c1, sum(g::numeric) as c2, count(*) as c3
-  from generate_series(0, 19999) g
-  group by g%10000;
+  from agg_data_20k group by g%10000;
 
 create table agg_group_2 as
 select * from
@@ -1082,7 +1088,7 @@ select * from
     select (g/2)::numeric as c1,
            array_agg(g::numeric) as c2,
 	   count(*) as c3
-    from generate_series(0, 1999) g
+    from agg_data_2k
     where g < r.a
     group by g/2) as s;
 
@@ -1090,13 +1096,11 @@ set jit_above_cost to default;
 
 create table agg_group_3 as
 select (g/2)::numeric as c1, sum(7::int4) as c2, count(*) as c3
-  from generate_series(0, 1999) g
-  group by g/2;
+  from agg_data_2k group by g/2;
 
 create table agg_group_4 as
 select (g/2)::numeric as c1, array_agg(g::numeric) as c2, count(*) as c3
-  from generate_series(0, 1999) g
-  group by g/2;
+  from agg_data_2k group by g/2;
 
 -- Produce results with hash aggregation
 
@@ -1107,13 +1111,11 @@ set jit_above_cost = 0;
 
 explain (costs off)
 select g%10000 as c1, sum(g::numeric) as c2, count(*) as c3
-  from generate_series(0, 19999) g
-  group by g%10000;
+  from agg_data_20k group by g%10000;
 
 create table agg_hash_1 as
 select g%10000 as c1, sum(g::numeric) as c2, count(*) as c3
-  from generate_series(0, 19999) g
-  group by g%10000;
+  from agg_data_20k group by g%10000;
 
 create table agg_hash_2 as
 select * from
@@ -1122,7 +1124,7 @@ select * from
     select (g/2)::numeric as c1,
            array_agg(g::numeric) as c2,
 	   count(*) as c3
-    from generate_series(0, 1999) g
+    from agg_data_2k
     where g < r.a
     group by g/2) as s;
 
@@ -1130,13 +1132,11 @@ set jit_above_cost to default;
 
 create table agg_hash_3 as
 select (g/2)::numeric as c1, sum(7::int4) as c2, count(*) as c3
-  from generate_series(0, 1999) g
-  group by g/2;
+  from agg_data_2k group by g/2;
 
 create table agg_hash_4 as
 select (g/2)::numeric as c1, array_agg(g::numeric) as c2, count(*) as c3
-  from generate_series(0, 1999) g
-  group by g/2;
+  from agg_data_2k group by g/2;
 
 set enable_sort = true;
 set work_mem to default;

src/test/regress/sql/groupingsets.sql

@@ -444,10 +444,17 @@ select v||'a', case when grouping(v||'a') = 1 then 1 else 0 end, count(*)
 --
 -- Compare results between plans using sorting and plans using hash
 -- aggregation. Force spilling in both cases by setting work_mem low
+-- and altering the statistics.
 --
 
-
+create table gs_data_1 as
+select g%1000 as g1000, g%100 as g100, g%10 as g10, g
+   from generate_series(0,1999) g;
+
+analyze gs_data_1;
+alter table gs_data_1 set (autovacuum_enabled = 'false');
+update pg_class set reltuples = 10 where relname='gs_data_1';
+
 SET work_mem='64kB';
 
 -- Produce results with sorting.
@@ -456,16 +463,12 @@ set enable_hashagg = false;
 set jit_above_cost = 0;
 
 explain (costs off)
-select g100, g10, sum(g::numeric), count(*), max(g::text) from
-  (select g%1000 as g1000, g%100 as g100, g%10 as g10, g
-   from generate_series(0,1999) g) s
-group by cube (g1000, g100,g10);
+select g100, g10, sum(g::numeric), count(*), max(g::text)
+from gs_data_1 group by cube (g1000, g100,g10);
 
 create table gs_group_1 as
-select g100, g10, sum(g::numeric), count(*), max(g::text) from
-  (select g%1000 as g1000, g%100 as g100, g%10 as g10, g
-   from generate_series(0,1999) g) s
-group by cube (g1000, g100,g10);
+select g100, g10, sum(g::numeric), count(*), max(g::text)
+from gs_data_1 group by cube (g1000, g100,g10);
 
 -- Produce results with hash aggregation.
 
@@ -473,17 +476,12 @@ set enable_hashagg = true;
 set enable_sort = false;
 
 explain (costs off)
-select g100, g10, sum(g::numeric), count(*), max(g::text) from
-  (select g%1000 as g1000, g%100 as g100, g%10 as g10, g
-   from generate_series(0,1999) g) s
-group by cube (g1000, g100,g10);
+select g100, g10, sum(g::numeric), count(*), max(g::text)
+from gs_data_1 group by cube (g1000, g100,g10);
 
 create table gs_hash_1 as
-select g100, g10, sum(g::numeric), count(*), max(g::text) from
-  (select g%1000 as g1000, g%100 as g100, g%10 as g10, g
-   from generate_series(0,1999) g) s
-group by cube (g1000, g100,g10);
-
+select g100, g10, sum(g::numeric), count(*), max(g::text)
+from gs_data_1 group by cube (g1000, g100,g10);
 
 set enable_sort = true;
 set work_mem to default;
@@ -497,6 +495,4 @@ set work_mem to default;
 drop table gs_group_1;
 drop table gs_hash_1;
 
-SET enable_groupingsets_hash_disk TO DEFAULT;
-
 -- end