pgsql: pg_plan_advice: Fix a bug when a subquery is pruned away entirel

pg_plan_advice: Fix a bug when a subquery is pruned away entirely.

If a subquery is proven empty, and if that subquery contained a
semijoin, and if making one side or the other of that semijoin
unique and performing an inner join was a possible strategy, then
the previous code would fail with ERROR: no rtoffset for plan %s
when attempting to generate advice. Fix that.

Reported-by: Alexander Lakhin <exclusion(at)gmail(dot)com>
Discussion: http://postgr.es/m/CA+TgmobOOmmXSJz3e+cjTY-bA1+W0dqVDqzxUBEvGtW62whYGg@mail.gmail.com

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/0f93ebb3112d562fe54b685d0ca08c5ba1b41467

Modified Files
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contrib/pg_plan_advice/expected/semijoin.out | 17 +++++++++++++++++
contrib/pg_plan_advice/pgpa_planner.c | 28 ++++++++++++++++------------
contrib/pg_plan_advice/sql/semijoin.sql | 9 +++++++++
3 files changed, 42 insertions(+), 12 deletions(-)