Improve the performance of the standby server when dropping tables on the primary server

Hi,

The attached patch changes data structure storing unowned SMgrRelation objects
from list structure to hash structure.
The reason why I change it is that list structure very slowly removes a node.
And list structure takes longer time to remove a node than hash structure.

The problem was reported in BUG #14575.
https://www.postgresql.org/message-id/20170303023246.25054.66379@wrigleys.postgresql.org

In my customer's case, the standby server was delayed more than 20 minites
when dropping many table at once.

- Performance check

I confirmed the performance of dropping tables by the following method.

1. Set up a synchronous streaming replication environment.
And set synchronous_commit = remote_apply in postgresql.conf.

2. Create 100,000 tables (tbl1, tbl2, ... , tbl100000).
And insert one row in each table.

3. Measure the time to drop 50 tables by psql

$ time psql -d ${DATABSE} -p ${PORT} -f drop.sql

drop.sql
--
begin;
drop table tbl1;
drop table tbl2;
...
drop table tbl50;
commit;
--

Result:
without this patch
real 0m3.734s
user 0m0.003s
sys 0m0.005s

with this patch
real 0m1.292s
user 0m0.005s
sys 0m0.003s

Even in this case, we have improved considerably,
so I suggest you might approve it.

Regards,
Takashi Tokuda