pgsql: Fix semijoin unique-ification for child relations

Fix semijoin unique-ification for child relations

For a child relation, we should not assume that its parent's
unique-ified relation (or unique-ified path in v18) always exists.  In
cases where all RHS columns that need to be unique-ified are equated
to constants, the unique-ified relation/path for the parent table is
not built, as there are no columns left to unique-ify.  Failing to
account for this can result in a SIGSEGV crash during planning.

This patch checks whether the parent's unique-ified relation or path
exists and skips unique-ification of the child relation if it does
not.

Author: Richard Guo <guofenglinux@gmail.com>
Discussion: https://postgr.es/m/CAMbWs49MOdLW2c+qbLHHBt8VBu=4ONpM91D19=AWeW93eFUF6A@mail.gmail.com
Backpatch-through: 18

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/97b0f36bde9a08bc6f004438ff8fc0afbcb418c0

Modified Files
--------------
src/backend/optimizer/plan/planner.c |  9 ++++++
src/test/regress/expected/join.out   | 60 ++++++++++++++++++++++++++++++++++++
src/test/regress/sql/join.sql        | 37 ++++++++++++++++++++++
3 files changed, 106 insertions(+)