On Tue, Mar 13, 2018 at 4:14 AM, Tom Lane <tgl@sss.pgh.pa.us> wrote: > Mat Arye <mat@timescale.com> writes: >> So the use-case is an analytical query like > >> SELECT date_trunc('hour', time) AS MetricMinuteTs, AVG(value) as avg >> FROM hyper >> WHERE time >= '2001-01-04T00:00:00' AND time <= '2001-01-05T01:00:00' >> GROUP BY MetricMinuteTs >> ORDER BY MetricMinuteTs DESC; > >> Right now this query will choose a much-less-efficient GroupAggregate plan >> instead of a HashAggregate. It will choose this because it thinks the >> number of groups >> produced here is 9,000,000 because that's the number of distinct time >> values there are. >> But, because date_trunc "buckets" the values there will be about 24 groups >> (1 for each hour). > > While it would certainly be nice to have better behavior for that, > "add a hook so users who can write C can fix it by hand" doesn't seem > like a great solution. On top of the sheer difficulty of writing a > hook function, you'd have the problem that no pre-written hook could > know about all available functions. I think somehow we'd need a way > to add per-function knowledge, perhaps roughly like the protransform > feature.
Like cost associated with a function, we may associate mapping cardinality with a function. It tells how many distinct input values map to 1 output value. By input value, I mean input argument tuple. In Mat's case the mapping cardinality will be 12. The number of distinct values that function may output is estimated as number of estimated rows / mapping cardinality of that function.
I think this is complicated by the fact that the mapping cardinality is not a constant per function
but depends on the constant given as the first argument to the function and the granularity of the
underlying data (do you have a second-granularity or microsecond granularity). I actually think the logic for the
estimate here should be the (max(time)-min(time))/interval. I think to be general you need to allow functions on statistics to determine the estimate.
-- Best Wishes, Ashutosh Bapat EnterpriseDB Corporation The Postgres Database Company
