pgsql: Properly prepare varinfos in estimate_multivariate_bucketsize()

Properly prepare varinfos in estimate_multivariate_bucketsize()

To estimate with extended statistics, we need to clear the varnullingrels
field in the expression, and duplicates are not allowed in the GroupVarInfo
list. We might re-use add_unique_group_var(), but we don't do so for two
reasons.

1) We must keep the origin_rinfos list ordered exactly the same way as
varinfos.
2) add_unique_group_var() is designed for estimate_num_groups(), where a
larger number of groups is worse. While estimating the number of hash
buckets, we have the opposite: a lesser number of groups is worse.
Therefore, we don't have to remove "known equal" vars: the removed var
may valuably contribute to the multivariate statistics to grow the number
of groups.

This commit adds custom code to estimate_multivariate_bucketsize() to
initialize varinfos properly.

Reported-by: Robins Tharakan <tharakan(at)gmail(dot)com>
Discussion: https://postgr.es/m/18885-da51324078588253%40postgresql.org
Author: Andrei Lepikhov <lepihov(at)gmail(dot)com>
Reviewed-by: David Rowley <dgrowleyml(at)gmail(dot)com>
Reviewed-by: Tomas Vondra <tomas(at)vondra(dot)me>
Reviewed-by: Alexander Korotkov <aekorotkov(at)gmail(dot)com>

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/9f404d7922e8831dc49bfa225530ba5309900e4e

Modified Files
--------------
src/backend/utils/adt/selfuncs.c | 60 +++++++++++++++++++++++++++++++--
src/test/regress/expected/stats_ext.out | 28 +++++++++++++++
src/test/regress/sql/stats_ext.sql | 11 ++++++
3 files changed, 96 insertions(+), 3 deletions(-)