create extension pg_dropcache;
create extension pg_buffercache;
--
create table t(x int);
insert into t select * from generate_series(1, 1000000);
select page_count > 4000 as page_count_gt_4000 from (
  select count(*) as page_count from pg_buffercache where relfilenode = pg_relation_filenode('t'::regclass)
) as foo;
 page_count_gt_4000 
--------------------
 t
(1 row)

select pg_drop_rel_cache('t'::regclass);
 pg_drop_rel_cache 
-------------------
 
(1 row)

select count(*) from pg_buffercache where relfilenode = pg_relation_filenode('t'::regclass); 
 count 
-------
     0
(1 row)

--
create table t1(x int);
create table t2(x int primary key);
insert into t1 select * from generate_series(1, 1000000);
insert into t2 select * from generate_series(1, 1000000);
select page_count_t1 > 4000 as page_count_t1_gt_4000 from (
  select count(*) as page_count_t1 from pg_buffercache where relfilenode = pg_relation_filenode('t1'::regclass)
) as foo;
 page_count_t1_gt_4000 
-----------------------
 t
(1 row)

select page_count_t2 > 4000 as page_count_t2_gt_4000 from (
  select count(*) as page_count_t2 from pg_buffercache where relfilenode = pg_relation_filenode('t2'::regclass)
) as foo;
 page_count_t2_gt_4000 
-----------------------
 t
(1 row)

select page_count_t2_pkey > 40 as page_count_t2_pkey_gt_40 from (
  select count(*) as page_count_t2_pkey from pg_buffercache where relfilenode = pg_relation_filenode('t2_pkey'::regclass)
) as foo;
 page_count_t2_pkey_gt_40 
--------------------------
 t
(1 row)

select pg_drop_cache();
 pg_drop_cache 
---------------
 
(1 row)

select count(*) from pg_buffercache where relfilenode = pg_relation_filenode('t1'::regclass); 
 count 
-------
     0
(1 row)

select count(*) from pg_buffercache where relfilenode = pg_relation_filenode('t2'::regclass); 
 count 
-------
     0
(1 row)

select count(*) from pg_buffercache where relfilenode = pg_relation_filenode('t2_pkey'::regclass);
 count 
-------
     0
(1 row)