pg_stat_have_stats() returns true for dropped indexes (or for index creation transaction rolled back)

Hi hackers,

While working on the relation stats split into table and index stats 
[1], I noticed that currently pg_stat_have_stats() returns true for 
dropped indexes (or for index creation transaction rolled back).

Example:

postgres=# create table bdt as select a from generate_series(1,1000) a;
SELECT 1000
postgres=# create index bdtidx on bdt(a);
CREATE INDEX
postgres=# select * from bdt where a = 30;
  a
----
  30
(1 row)

postgres=# SELECT 'bdtidx'::regclass::oid;
   oid
-------
  16395
(1 row)

postgres=# select pg_stat_have_stats('relation', 5, 16395);
  pg_stat_have_stats
--------------------
  t
(1 row)

postgres=# drop index bdtidx;
DROP INDEX
postgres=# select pg_stat_have_stats('relation', 5, 16395);
  pg_stat_have_stats
--------------------
  t
(1 row)


Please find attached a patch proposal to fix it.

It does contain additional calls to pgstat_create_relation() and 
pgstat_drop_relation() as well as additional TAP tests.

[1]: 
https://www.postgresql.org/message-id/5bfcf1a5-4224-9324-594b-725e704c95b1%40amazon.com

Regards,

-- 
Bertrand Drouvot
Amazon Web Services: https://aws.amazon.com