Andy Fan <zhihui.fan1213@gmail.com> writes:
> On Fri, Aug 21, 2020 at 5:51 AM Andrew Gierth <andrew@tao11.riddles.org.uk>
> wrote:
>> Unless I'm missing something, it should be safe to reference output
>> columns that are not mentioned in any grouping set,
> I think such columns usually are aggregation expr, If we want to push down
> a qual which reference to an aggregation expr, we have to push down
> to having cause, However I am not sure such pushing down really helps.
Well, they can either be aggregates, or functions of the grouping
columns. You're right that there's not much we can do (today) with
restrictions on aggregate outputs, but there can be value in pushing
down restrictions on the other sort.
As an example, consider the regression database's tenk1 table, and
for argument's sake add
regression=# create index on tenk1 (abs(hundred));
CREATE INDEX
Then we can get
regression=# explain select * from (select hundred, ten, abs(hundred) a, count(*) c from tenk1 group by 1,2) ss where a
=42;
QUERY PLAN
------------------------------------------------------------------------------------
HashAggregate (cost=225.98..227.18 rows=96 width=20)
Group Key: tenk1.hundred, tenk1.ten
-> Bitmap Heap Scan on tenk1 (cost=5.06..225.23 rows=100 width=8)
Recheck Cond: (abs(hundred) = 42)
-> Bitmap Index Scan on tenk1_abs_idx (cost=0.00..5.04 rows=100 width=0)
Index Cond: (abs(hundred) = 42)
(6 rows)
which is a lot cheaper than the pure seqscan you get with no pushed-down
condition.
One thing that I find curious is that if I alter this example to use
grouping sets, say
regression=# explain select * from (select hundred, ten, abs(hundred) a, count(*) c from tenk1 group by grouping sets
(1,2))ss where a = 42;
QUERY PLAN
-----------------------------------------------------------------
HashAggregate (cost=495.00..546.65 rows=2 width=20)
Hash Key: tenk1.hundred
Hash Key: tenk1.ten
Filter: (abs(tenk1.hundred) = 42)
-> Seq Scan on tenk1 (cost=0.00..445.00 rows=10000 width=8)
(5 rows)
i.e. it's not seeing the abs() condition as pushable below the
aggregation. I'm not quite sure if that's a necessary restriction
or a missed optimization.
regards, tom lane