pgsql: Clean up remove_rel_from_query() after self-join elimination com

Clean up remove_rel_from_query() after self-join elimination commit

The self-join elimination (SJE) commit grafted self-join removal onto
remove_rel_from_query(), which was originally written for left-join
removal only. This resulted in several issues:

- Comments throughout remove_rel_from_query() still assumed only
left-join removal, making the code misleading.

- ChangeVarNodesExtended was called on phv->phexpr with subst=-1
during left-join removal, which is pointless and confusing since any
surviving PHV shouldn't reference the removed rel.

- phinfo->ph_lateral was adjusted for left-join removal, which is
unnecessary since the removed relid cannot appear in ph_lateral for
outer joins.

- The comment about attr_needed reconstruction was in
remove_rel_from_query(), but the actual rebuild is performed by the
callers.

- EquivalenceClass processing in remove_rel_from_query() is redundant
for self-join removal, since the caller (remove_self_join_rel)
already handles ECs via update_eclasses().

- In remove_self_join_rel(), ChangeVarNodesExtended was called on
root->processed_groupClause, which contains SortGroupClause nodes
that have no Var nodes to rewrite. The accompanying comment
incorrectly mentioned "HAVING clause".

This patch fixes all these issues, clarifying the separation between
left-join removal and self-join elimination code paths within
remove_rel_from_query(). The resulting code is also better structured
for adding new types of join removal (such as inner-join removal) in
the future.

Author: Richard Guo <guofenglinux(at)gmail(dot)com>
Reviewed-by: Tender Wang <tndrwang(at)gmail(dot)com>
Reviewed-by: Andrei Lepikhov <lepihov(at)gmail(dot)com>
Reviewed-by: wenhui qiu <qiuwenhuifx(at)gmail(dot)com>
Discussion: https://postgr.es/m/CAMbWs48JC4OVqE=3gMB6se2WmRNNfMyFyYxm-09vgpm+Vwe8Hg@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/20efbdffeb6418afa13d6c8457054735d11c7e3a

Modified Files
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src/backend/optimizer/plan/analyzejoins.c | 453 +++++++++++++++---------------
1 file changed, 231 insertions(+), 222 deletions(-)