pgsql: Fix thinko in estimate_num_groups

Fix thinko in estimate_num_groups

The code for the reworked n-distinct estimation on commit 7b504eb282 was
written differently in a previous version of the patch, prior to commit;
on rewriting it, we missed updating an initializer.  This caused the
code to (mistakenly) apply a fudge factor even in the case where a
single value is applied, leading to incorrect results.

This means that the 'relvarcount' variable name is now wrong.  Add a
comment to try and make the situation clearer, and remove an incorrect
comment I added.

Problem noticed, and code patch, by Tomas Vondra.  Additional commentary
by Álvaro.

Branch
------
master

Details
-------
http://git.postgresql.org/pg/commitdiff/1f171a1803c28d3ae24636c9ca3352ec82c39e5f

Modified Files
--------------
src/backend/utils/adt/selfuncs.c | 8 ++++++--
1 file changed, 6 insertions(+), 2 deletions(-)