Could not control access plan on REL95_1_1_8 in one use case about pg_hint_plan

[horiguti]

The repor doesn't work for me, with any ordering of the two extensions in shared_preload_libraries and both -f or not. Do you suppose some other conditions for the behavior?

[higuchi-daisuke]

Hi, I'm sorry for confusing you.
I could not reproduce this issue even if I follow the procedure in my first post..
So, I write the simple procedure to reproduce. I use pg_hint_plan REL95_1_1_8 and PostgreSQL 9.5.21.

1. Initial setup by following my first post.
2. Execute following command:
   psql -c "EXPLAIN SELECT /*+ MergeJoin(t1 t2) */ T1.C1,T2.C1 FROM S1.T1,S1.T2 WHERE T1.C1=T2.C1;"

I confirm not Merge Join but Hash Join from explain information of 2.

[ A little investigation ]
When new session starts, pg_dbms_stats needs to search "locked statistics" tables, I think.
If so, this behavior is similar to the following comment of commit 000c325.

Hints lose effect when pg_dbms_stats searches "locked statistics" tables 
while planning the target query.

[horiguti]

OK, I confirmed that when both pg_hint_plan and pg_dbms_stats are preloaded, running the given query as the first query in a session doesn't honor the hint. Some digging on the repository told me that a change committed fife years ago (9.4 era) have broken the previous behavior.
Fixed it. Patched from PG94 up to master (PG12).

4e63a22