On 9/20/25 18:51, Tom Lane wrote:
I concluded that maybe I was overthinking this part. We only really
need to check the rowcount estimate, since the indexscan cost estimate
is already okay. And stats_ext.sql seems to have gotten away with
assuming that rowcount estimates are reliably reproducible on
not-too-large tables. This bug affects use of extended statistics
too, so a test using those is good enough to show it's fixed; we don't
really need to use an expression index for the purpose. So, I added a
test case in stats_ext.sql and pushed it.
OK, great! Thanks for the detailed explanation.
Regarding extended statistics, it's unfortunate that they cannot be used
in this case. Multivariate MCV statistics work as long as the number of
rows in the table is reasonably small (100K) and the STATISTICS value
for a column is high enough (it doesn't matter which column, because
only the sample size matters).
I wonder if this is an argument in favour of decoupling the sample size
and the precision of the statistics. Here, we basically want the sample
size to be as big as the table in order to include the few (NULL,
WARNING) values.
Or maybe we need a different kind of extended statistics?
