Reduce initial size of RelfilenodeMapHash.
Reduce initial size of RelfilenodeMapHash.

A test case provided by Mathieu Fenniak shows that hash_seq_search'ing
this hashtable can consume a very significant amount of overhead during
logical decoding, which triggers frequent cache invalidation.  Testing
suggests that the actual population of the hashtable is often no more
than a few dozen entries, so we can cut the overhead just by dropping
the initial number of buckets down from 1024 --- I chose to cut it to 64.
(In situations where we do have a significant number of entries, we
shouldn't get any real penalty from doing this, as the dynahash.c code
will resize the hashtable automatically.)

This gives a further factor-of-two savings in Mathieu's test case.
That may be overly optimistic for real-world benefit, as real cases
may have larger average table populations, but it's hard to see it
turning into a net negative for any workload.

Back-patch to 9.4 where relfilenodemap.c was introduced.

Discussion: b14dda2febfb74bad66d8ef825016dd7e1c85571@mail.gmail.com

50wu/gpdb,lisakowen/gpdb,adam8157/gpdb,adam8157/gpdb,greenplum-db/gpdb,lisakowen/gpdb,50wu/gpdb,lisakowen/gpdb,xinzweb/gpdb,greenplum-db/gpdb,50wu/gpdb,lisakowen/gpdb,xinzweb/gpdb,xinzweb/gpdb,xinzweb/gpdb,greenplum-db/gpdb,adam8157/gpdb,xinzweb/gpdb,greenplum-db/gpdb,adam8157/gpdb,lisakowen/gpdb,greenplum-db/gpdb,50wu/gpdb,adam8157/gpdb,greenplum-db/gpdb,xinzweb/gpdb,50wu/gpdb,adam8157/gpdb,50wu/gpdb,lisakowen/gpdb,50wu/gpdb,lisakowen/gpdb,adam8157/gpdb,adam8157/gpdb,greenplum-db/gpdb,lisakowen/gpdb,xinzweb/gpdb,greenplum-db/gpdb,50wu/gpdb,xinzweb/gpdb