On 1 February 2018 at 17:49, Robert Haas <robertmhaas@gmail.com> wrote:
> One point which I want to emphasize is that the length of the MCV list
> bounds the estimated frequency of non-MCVs in two ways: no non-MCV is
> ever thought to be more frequent than the least-common MCVs, and
> however many non-MCVs we think we have (probably fewer than we
> actually have) have to fit into whatever percentage of the table is
> consumed by MCVs. This would be less important if we had reliable
> n_distinct estimates, but we don't. So, even throwing things into the
> MCV list that are no more common than the average item can improve
> planning in some cases.
>
That's a good point, and a nice explanation. I think that lends more
weight to the argument that we should be including as many MCVs as
possible, provided there's enough evidence to justify their inclusion.
Regards,
Dean
