On Tue, Mar 13, 2018 at 8:55 PM, Mat Arye <m...@timescale.com> wrote: >> >> 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. >
I think my solution was quite short-sighted. You are right. We need a function taking statistics about the input argument as input and output the statistics about the output. The planner can then use this statistics to arrive at various estimates. -- Best Wishes, Ashutosh Bapat EnterpriseDB Corporation The Postgres Database Company
