pgsql: pg_plan_advice: Fix another unique-semijoin bug.

pg_plan_advice: Fix another unique-semijoin bug.

This one occurs when an outer join appears beneath the made-unique
side of a semijoin. The issue is that join RTEs are not featured
out of sj_unique_rels entries. Fix, and add a test case.

Reported-by: Alexander Lakhin <exclusion(at)gmail(dot)com>
Analyzed-by: Tender Wang <tndrwang(at)gmail(dot)com>
Discussion: http://postgr.es/m/c0c63979-43c2-4424-8fe8-56949934c9d8@gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/4321dcad475bc7cbb4efe18c0b0652a0506e8348

Modified Files
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contrib/pg_plan_advice/expected/semijoin.out | 32 ++++++++++++++++++++++++++++
contrib/pg_plan_advice/pgpa_planner.c | 13 +++++++++--
contrib/pg_plan_advice/sql/semijoin.sql | 9 ++++++++
3 files changed, 52 insertions(+), 2 deletions(-)