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Support hashed aggregation with grouping sets.
This extends the Aggregate node with two new features: HashAggregate can now run multiple hashtables concurrently, and a new strategy MixedAggregate populates hashtables while doing sorted grouping. The planner will now attempt to save as many sorts as possible when planning grouping sets queries, while not exceeding work_mem for the estimated combined sizes of all hashtables used. No SQL-level changes are required. There should be no user-visible impact other than the new EXPLAIN output and possible changes to result ordering when ORDER BY was not used (which affected a few regression tests). The enable_hashagg option is respected. Author: Andrew Gierth Reviewers: Mark Dilger, Andres Freund Discussion: https://postgr.es/m/87vatszyhj.fsf@news-spur.riddles.org.uk
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‎contrib/postgres_fdw/expected/postgres_fdw.out

Lines changed: 33 additions & 28 deletions
Original file line numberDiff line numberDiff line change
@@ -3276,16 +3276,19 @@ select sum(q.a), count(q.b) from ft4 left join (select 13, avg(ft1.c1), sum(ft2.
32763276
-- Grouping sets
32773277
explain (verbose, costs off)
32783278
select c2, sum(c1) from ft1 where c2 < 3 group by rollup(c2) order by 1 nulls last;
3279-
QUERY PLAN
3280-
---------------------------------------------------------------------------------------------------
3281-
GroupAggregate
3282-
Output: c2, sum(c1)
3283-
Group Key: ft1.c2
3284-
Group Key: ()
3285-
-> Foreign Scan on public.ft1
3286-
Output: c2, c1
3287-
Remote SQL: SELECT "C 1", c2 FROM "S 1"."T 1" WHERE ((c2 < 3)) ORDER BY c2 ASC NULLS LAST
3288-
(7 rows)
3279+
QUERY PLAN
3280+
------------------------------------------------------------------------------
3281+
Sort
3282+
Output: c2, (sum(c1))
3283+
Sort Key: ft1.c2
3284+
-> MixedAggregate
3285+
Output: c2, sum(c1)
3286+
Hash Key: ft1.c2
3287+
Group Key: ()
3288+
-> Foreign Scan on public.ft1
3289+
Output: c2, c1
3290+
Remote SQL: SELECT "C 1", c2 FROM "S 1"."T 1" WHERE ((c2 < 3))
3291+
(10 rows)
32893292

32903293
select c2, sum(c1) from ft1 where c2 < 3 group by rollup(c2) order by 1 nulls last;
32913294
c2 | sum
@@ -3298,16 +3301,19 @@ select c2, sum(c1) from ft1 where c2 < 3 group by rollup(c2) order by 1 nulls la
32983301

32993302
explain (verbose, costs off)
33003303
select c2, sum(c1) from ft1 where c2 < 3 group by cube(c2) order by 1 nulls last;
3301-
QUERY PLAN
3302-
---------------------------------------------------------------------------------------------------
3303-
GroupAggregate
3304-
Output: c2, sum(c1)
3305-
Group Key: ft1.c2
3306-
Group Key: ()
3307-
-> Foreign Scan on public.ft1
3308-
Output: c2, c1
3309-
Remote SQL: SELECT "C 1", c2 FROM "S 1"."T 1" WHERE ((c2 < 3)) ORDER BY c2 ASC NULLS LAST
3310-
(7 rows)
3304+
QUERY PLAN
3305+
------------------------------------------------------------------------------
3306+
Sort
3307+
Output: c2, (sum(c1))
3308+
Sort Key: ft1.c2
3309+
-> MixedAggregate
3310+
Output: c2, sum(c1)
3311+
Hash Key: ft1.c2
3312+
Group Key: ()
3313+
-> Foreign Scan on public.ft1
3314+
Output: c2, c1
3315+
Remote SQL: SELECT "C 1", c2 FROM "S 1"."T 1" WHERE ((c2 < 3))
3316+
(10 rows)
33113317

33123318
select c2, sum(c1) from ft1 where c2 < 3 group by cube(c2) order by 1 nulls last;
33133319
c2 | sum
@@ -3320,20 +3326,19 @@ select c2, sum(c1) from ft1 where c2 < 3 group by cube(c2) order by 1 nulls last
33203326

33213327
explain (verbose, costs off)
33223328
select c2, c6, sum(c1) from ft1 where c2 < 3 group by grouping sets(c2, c6) order by 1 nulls last, 2 nulls last;
3323-
QUERY PLAN
3324-
-------------------------------------------------------------------------------------------------------------
3329+
QUERY PLAN
3330+
----------------------------------------------------------------------------------
33253331
Sort
33263332
Output: c2, c6, (sum(c1))
33273333
Sort Key: ft1.c2, ft1.c6
3328-
-> GroupAggregate
3334+
-> HashAggregate
33293335
Output: c2, c6, sum(c1)
3330-
Group Key: ft1.c2
3331-
Sort Key: ft1.c6
3332-
Group Key: ft1.c6
3336+
Hash Key: ft1.c2
3337+
Hash Key: ft1.c6
33333338
-> Foreign Scan on public.ft1
33343339
Output: c2, c6, c1
3335-
Remote SQL: SELECT "C 1", c2, c6 FROM "S 1"."T 1" WHERE ((c2 < 3)) ORDER BY c2 ASC NULLS LAST
3336-
(11 rows)
3340+
Remote SQL: SELECT "C 1", c2, c6 FROM "S 1"."T 1" WHERE ((c2 < 3))
3341+
(10 rows)
33373342

33383343
select c2, c6, sum(c1) from ft1 where c2 < 3 group by grouping sets(c2, c6) order by 1 nulls last, 2 nulls last;
33393344
c2 | c6 | sum

‎src/backend/commands/explain.c

Lines changed: 21 additions & 4 deletions
Original file line numberDiff line numberDiff line change
@@ -1015,6 +1015,10 @@ ExplainNode(PlanState *planstate, List *ancestors,
10151015
pname = "HashAggregate";
10161016
strategy = "Hashed";
10171017
break;
1018+
case AGG_MIXED:
1019+
pname = "MixedAggregate";
1020+
strategy = "Mixed";
1021+
break;
10181022
default:
10191023
pname = "Aggregate ???";
10201024
strategy = "???";
@@ -1978,6 +1982,19 @@ show_grouping_set_keys(PlanState *planstate,
19781982
ListCell *lc;
19791983
List *gsets = aggnode->groupingSets;
19801984
AttrNumber *keycols = aggnode->grpColIdx;
1985+
const char *keyname;
1986+
const char *keysetname;
1987+
1988+
if (aggnode->aggstrategy == AGG_HASHED || aggnode->aggstrategy == AGG_MIXED)
1989+
{
1990+
keyname = "Hash Key";
1991+
keysetname = "Hash Keys";
1992+
}
1993+
else
1994+
{
1995+
keyname = "Group Key";
1996+
keysetname = "Group Keys";
1997+
}
19811998

19821999
ExplainOpenGroup("Grouping Set", NULL, true, es);
19832000

@@ -1992,7 +2009,7 @@ show_grouping_set_keys(PlanState *planstate,
19922009
es->indent++;
19932010
}
19942011

1995-
ExplainOpenGroup("Group Keys", "Group Keys", false, es);
2012+
ExplainOpenGroup(keysetname, keysetname, false, es);
19962013

19972014
foreach(lc, gsets)
19982015
{
@@ -2016,12 +2033,12 @@ show_grouping_set_keys(PlanState *planstate,
20162033
}
20172034

20182035
if (!result && es->format == EXPLAIN_FORMAT_TEXT)
2019-
ExplainPropertyText("Group Key", "()", es);
2036+
ExplainPropertyText(keyname, "()", es);
20202037
else
2021-
ExplainPropertyListNested("Group Key", result, es);
2038+
ExplainPropertyListNested(keyname, result, es);
20222039
}
20232040

2024-
ExplainCloseGroup("Group Keys", "Group Keys", false, es);
2041+
ExplainCloseGroup(keysetname, keysetname, false, es);
20252042

20262043
if (sortnode && es->format == EXPLAIN_FORMAT_TEXT)
20272044
es->indent--;

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