pg_buffercache: Add pg_buffercache_os_pages

ba2a3c2302 has added a way to check if a buffer is spread across
multiple pages with some NUMA information, via a new view
pg_buffercache_numa that depends on pg_buffercache_numa_pages(), a SQL
function.  These can only be queried when support for libnuma exists,
generating an error if not.

However, it can be useful to know how shared buffers and OS pages map
when NUMA is not supported or not available.  This commit expands the
capabilities around pg_buffercache_numa:
- pg_buffercache_numa_pages() is refactored as an internal function,
able to optionally process NUMA.  Its SQL definition prior to this
commit is still around to ensure backward-compatibility with v1.6.
- A SQL function called pg_buffercache_os_pages() is added, able to work
with or without NUMA.
- The view pg_buffercache_numa is redefined to use
pg_buffercache_os_pages().
- A new view is added, called pg_buffercache_os_pages.  This ignores
NUMA for its result processing, for a better efficiency.

The implementation is done so as there is no code duplication between
the NUMA and non-NUMA views/functions, relying on one internal function
that does the job for all of them.  The module is bumped to v1.7.

Author: Bertrand Drouvot <bertranddrouvot.pg@gmail.com>
Reviewed-by: Mircea Cadariu <cadariu.mircea@gmail.com>
Reviewed-by: Michael Paquier <michael@paquier.xyz>
Discussion: https://postgr.es/m/Z/fFA2heH6lpSLlt@ip-10-97-1-34.eu-west-3.compute.internal

contrib/pg_buffercache/sql/pg_buffercache.sql

@@ -5,6 +5,12 @@ select count(*) = (select setting::bigint
                    where name = 'shared_buffers')
 from pg_buffercache;
 
+-- For pg_buffercache_os_pages, we expect at least one entry for each buffer
+select count(*) >= (select setting::bigint
+                    from pg_settings
+                    where name = 'shared_buffers')
+from pg_buffercache_os_pages;
+
 select buffers_used + buffers_unused > 0,
         buffers_dirty <= buffers_used,
         buffers_pinned <= buffers_used
@@ -16,6 +22,7 @@ SELECT count(*) > 0 FROM pg_buffercache_usage_counts() WHERE buffers >= 0;
 -- having to create a dedicated user, use the pg_database_owner pseudo-role.
 SET ROLE pg_database_owner;
 SELECT * FROM pg_buffercache;
+SELECT * FROM pg_buffercache_os_pages;
 SELECT * FROM pg_buffercache_pages() AS p (wrong int);
 SELECT * FROM pg_buffercache_summary();
 SELECT * FROM pg_buffercache_usage_counts();
@@ -24,6 +31,7 @@ RESET role;
 -- Check that pg_monitor is allowed to query view / function
 SET ROLE pg_monitor;
 SELECT count(*) > 0 FROM pg_buffercache;
+SELECT count(*) > 0 FROM pg_buffercache_os_pages;
 SELECT buffers_used + buffers_unused > 0 FROM pg_buffercache_summary();
 SELECT count(*) > 0 FROM pg_buffercache_usage_counts();
 RESET role;