On Tue, Mar 13, 2018 at 8:55 PM, Mat Arye <mat@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