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

>>>>> "Andrew" == Andrew Gierth <andrew@tao11.riddles.org.uk> writes:
>>>>> "Tom" == Tom Lane <tgl@sss.pgh.pa.us> writes:
Tom> What I did in the patch is to scale the formerly fixed "-1.0"Tom> stadistinct estimate to discount the fraction of
nullswe found.
 
Andrew> This seems quite dubious to me. stadistinct representing onlyAndrew> the non-null values seems to me to be
substantiallymore usefulAndrew> and less confusing; it should be up to consumers to takeAndrew> stanullfrac into
account(in general they already do) since inAndrew> many cases we explicitly do _not_ want to count nulls.
 

Hm. I am wrong about this, since it's the fact that consumers are taking
stanullfrac into account that makes the value wrong in the first place.
For example, if a million-row table has stanullfrac=0.9 and
stadistinct=-1, then get_variable_numdistinct is returning 1 million,
and (for example) var_eq_non_const divides 0.1 by that to give a
selectivity of 1 in 10 million, which is obviously wrong.

But I think the fix is still wrong, because it changes the meaning of
ALTER TABLE ... ALTER col SET (n_distinct=...)  in a non-useful way; it
is no longer possible to nail down a useful negative n_distinct value if
the null fraction of the column is variable. Would it not make more
sense to do the adjustment in get_variable_numdistinct, instead?

-- 
Andrew (irc:RhodiumToad)