Add a simple example of pgfincore+buffercache SQL

This query is useful to know how a relation is cached.

diff --git a/examples/buffercache_pgfincore.sql b/examples/buffercache_pgfincore.sql
new file mode 100644 (file)
index 0000000..be72b61
--- /dev/null
+++ b/examples/buffercache_pgfincore.sql
@@ -0,0 +1,47 @@
+with my_table as (
+  select oid
+       , relfilenode
+       , relname
+  from pg_class
+  where relname = 'pgbench_accounts'
+)
+, t as (
+  select generate_series(1, relpages) as g
+  from my_table
+  join pg_class using (relname)
+)
+, buf as (
+  select relblocknumber * 2 as bn -- Pgfincore use filesystem block size
+       , usagecount as c
+       , isdirty as d
+  from my_table
+  join pg_buffercache using (relfilenode)
+  where relforknumber = 0
+)
+, pgf as (
+  select (row_number() over (partition by c)) - 1 as bn -- pascal vs C
+          , c
+          , NULL as d
+  from (select unnest(
+                      string_to_array(
+                                      (pgfincore(my_table.oid, true)).databit::text, NULL
+                                 )
+               ) as c 
+        from my_table ) g
+)
+, fb as (
+   select pgf.bn as file_block_number
+               , buf.c as pgcache
+       , buf.d as pgdirty
+        , pgf.c as oscache
+        , pgf.d as osdirty
+  from buf 
+  right join pgf using (bn)
+  order by 1, 2, 3
+),
+res as (
+  select *
+  from fb
+)
+select row_to_json(res) -- use "res" CTE if no JSON datatype (pg < 9.2)
+from res;