Re: Bogus ANALYZE results for an otherwise-unique column with many nulls

On 5 August 2016 at 21:48, Tom Lane <tgl@sss.pgh.pa.us> wrote:
> OK, thanks.  What shall we do about Andreas' request to back-patch this?
> I'm personally willing to do it, but there is the old bugaboo of "maybe
> it will destabilize a plan that someone is happy with".
>

My inclination would be to back-patch it because arguably it's a
bug-fix -- at the very least the old behaviour didn't match the docs
for stadistinct:
     The number of distinct nonnull data values in the column.     A value greater than zero is the actual number of
distinctvalues.     A value less than zero is the negative of a multiplier for the number     of rows in the table; for
example,a column in which values appear about     twice on the average could be represented by
<structfield>stadistinct</>= -0.5.
 

Additionally, I think that example is misleading because it's only
really true if there are no null values in the column. Perhaps it
would help to have a more explicit example to illustrate how nulls
affect stadistinct, for example:
     ... for example, a column in which about 80% of the values are nonnull     and each nonnull value appears about
twiceon average could be     represented by <structfield>stadistinct</> = -0.4.
 

Regards,
Dean