pgsql: Fix usage of the parse tree for estimate_num_groups() in set ope

Fix usage of the parse tree for estimate_num_groups() in set operations

recurse_set_operations() uses the parse tree for the group number estimation,
because of the "varno 0" hack. At the same time 2489d76c49 made root->parse
and corresponding parent_root->simple_rte_array[]->subquery distinct copies
of the parse tree, while d3d55ce571 introduced self-join removal replacing
relid of removed relation only in one of the copies.

The present commit fixes this bug by making recurse_set_operations() call
estimate_num_groups() with the copy of the parse tree processed by self-join
removal.

In future, we may think about maintaining just one copy of the parse tree
and/or keeping removed relids as aliases.

Reported-by: Zuming Jiang
Bug: #18170
Discussion: https://postgr.es/m/flat/18170-f1d17bf9a0d58b24%40postgresql.org
Author: Richard Guo, Alexander Korotkov
Reviewed-by: Andrei Lepikhov

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/ec63622c03ff7be36d32c08aaa7d853856451488

Modified Files
--------------
src/backend/optimizer/prep/prepunion.c | 7 +++++--
src/test/regress/expected/join.out | 17 +++++++++++++++++
src/test/regress/sql/join.sql | 7 +++++++
3 files changed, 29 insertions(+), 2 deletions(-)