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Support hashed aggregation with grouping sets.
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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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@RhodiumToad
RhodiumToad committed Mar 27, 2017
1 parent f0a6046 commit b563594
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Showing 22 changed files with 2,544 additions and 594 deletions.
61 changes: 33 additions & 28 deletions contrib/postgres_fdw/expected/postgres_fdw.out
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Original file line number Diff line number Diff line change
Expand Up @@ -3276,16 +3276,19 @@ select sum(q.a), count(q.b) from ft4 left join (select 13, avg(ft1.c1), sum(ft2.
-- Grouping sets
explain (verbose, costs off)
select c2, sum(c1) from ft1 where c2 < 3 group by rollup(c2) order by 1 nulls last;
QUERY PLAN
---------------------------------------------------------------------------------------------------
GroupAggregate
Output: c2, sum(c1)
Group Key: ft1.c2
Group Key: ()
-> Foreign Scan on public.ft1
Output: c2, c1
Remote SQL: SELECT "C 1", c2 FROM "S 1"."T 1" WHERE ((c2 < 3)) ORDER BY c2 ASC NULLS LAST
(7 rows)
QUERY PLAN
------------------------------------------------------------------------------
Sort
Output: c2, (sum(c1))
Sort Key: ft1.c2
-> MixedAggregate
Output: c2, sum(c1)
Hash Key: ft1.c2
Group Key: ()
-> Foreign Scan on public.ft1
Output: c2, c1
Remote SQL: SELECT "C 1", c2 FROM "S 1"."T 1" WHERE ((c2 < 3))
(10 rows)

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

explain (verbose, costs off)
select c2, sum(c1) from ft1 where c2 < 3 group by cube(c2) order by 1 nulls last;
QUERY PLAN
---------------------------------------------------------------------------------------------------
GroupAggregate
Output: c2, sum(c1)
Group Key: ft1.c2
Group Key: ()
-> Foreign Scan on public.ft1
Output: c2, c1
Remote SQL: SELECT "C 1", c2 FROM "S 1"."T 1" WHERE ((c2 < 3)) ORDER BY c2 ASC NULLS LAST
(7 rows)
QUERY PLAN
------------------------------------------------------------------------------
Sort
Output: c2, (sum(c1))
Sort Key: ft1.c2
-> MixedAggregate
Output: c2, sum(c1)
Hash Key: ft1.c2
Group Key: ()
-> Foreign Scan on public.ft1
Output: c2, c1
Remote SQL: SELECT "C 1", c2 FROM "S 1"."T 1" WHERE ((c2 < 3))
(10 rows)

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

explain (verbose, costs off)
select c2, c6, sum(c1) from ft1 where c2 < 3 group by grouping sets(c2, c6) order by 1 nulls last, 2 nulls last;
QUERY PLAN
-------------------------------------------------------------------------------------------------------------
QUERY PLAN
----------------------------------------------------------------------------------
Sort
Output: c2, c6, (sum(c1))
Sort Key: ft1.c2, ft1.c6
-> GroupAggregate
-> HashAggregate
Output: c2, c6, sum(c1)
Group Key: ft1.c2
Sort Key: ft1.c6
Group Key: ft1.c6
Hash Key: ft1.c2
Hash Key: ft1.c6
-> Foreign Scan on public.ft1
Output: c2, c6, c1
Remote SQL: SELECT "C 1", c2, c6 FROM "S 1"."T 1" WHERE ((c2 < 3)) ORDER BY c2 ASC NULLS LAST
(11 rows)
Remote SQL: SELECT "C 1", c2, c6 FROM "S 1"."T 1" WHERE ((c2 < 3))
(10 rows)

select c2, c6, sum(c1) from ft1 where c2 < 3 group by grouping sets(c2, c6) order by 1 nulls last, 2 nulls last;
c2 | c6 | sum
Expand Down
25 changes: 21 additions & 4 deletions src/backend/commands/explain.c
View file
Original file line number Diff line number Diff line change
Expand Up @@ -1015,6 +1015,10 @@ ExplainNode(PlanState *planstate, List *ancestors,
pname = "HashAggregate";
strategy = "Hashed";
break;
case AGG_MIXED:
pname = "MixedAggregate";
strategy = "Mixed";
break;
default:
pname = "Aggregate ???";
strategy = "???";
Expand Down Expand Up @@ -1978,6 +1982,19 @@ show_grouping_set_keys(PlanState *planstate,
ListCell *lc;
List *gsets = aggnode->groupingSets;
AttrNumber *keycols = aggnode->grpColIdx;
const char *keyname;
const char *keysetname;

if (aggnode->aggstrategy == AGG_HASHED || aggnode->aggstrategy == AGG_MIXED)
{
keyname = "Hash Key";
keysetname = "Hash Keys";
}
else
{
keyname = "Group Key";
keysetname = "Group Keys";
}

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

Expand All @@ -1992,7 +2009,7 @@ show_grouping_set_keys(PlanState *planstate,
es->indent++;
}

ExplainOpenGroup("Group Keys", "Group Keys", false, es);
ExplainOpenGroup(keysetname, keysetname, false, es);

foreach(lc, gsets)
{
Expand All @@ -2016,12 +2033,12 @@ show_grouping_set_keys(PlanState *planstate,
}

if (!result && es->format == EXPLAIN_FORMAT_TEXT)
ExplainPropertyText("Group Key", "()", es);
ExplainPropertyText(keyname, "()", es);
else
ExplainPropertyListNested("Group Key", result, es);
ExplainPropertyListNested(keyname, result, es);
}

ExplainCloseGroup("Group Keys", "Group Keys", false, es);
ExplainCloseGroup(keysetname, keysetname, false, es);

if (sortnode && es->format == EXPLAIN_FORMAT_TEXT)
es->indent--;
Expand Down
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