Unable to run pg_buffercache_pages() function

Question

I am trying to explore pg_buffercache extension and facing error while using pg_buffercache_pages() function. Errors are as follow :

test=# SELECT pg_buffercache_pages();
ERROR:  return type must be a row type

test=# SELECT * FROM pg_buffercache_pages();
ERROR:  a column definition list is required for functions returning "record"
LINE 1: SELECT * FROM pg_buffercache_pages();

                      ^

-- taken from .sql file of regress test
test=# SELECT * FROM pg_buffercache_pages() AS p (wrong int);
ERROR:  incorrect number of output arguments

I have confirmed that I am logged in as a superuser, meanwhile I have gone through the documentation but couldn't find how to run this function.

Answer 1

This function is not intended to be used directly, but via the pg_buffercache view. If you look at the definition of that view, you will see how it is intended to be called:

\d+ pg_buffercache 
                             View "public.pg_buffercache"
      Column      │   Type   │ Collation │ Nullable │ Default │ Storage │ Description 
══════════════════╪══════════╪═══════════╪══════════╪═════════╪═════════╪═════════════
 bufferid         │ integer  │           │          │         │ plain   │ 
 relfilenode      │ oid      │           │          │         │ plain   │ 
 reltablespace    │ oid      │           │          │         │ plain   │ 
 reldatabase      │ oid      │           │          │         │ plain   │ 
 relforknumber    │ smallint │           │          │         │ plain   │ 
 relblocknumber   │ bigint   │           │          │         │ plain   │ 
 isdirty          │ boolean  │           │          │         │ plain   │ 
 usagecount       │ smallint │           │          │         │ plain   │ 
 pinning_backends │ integer  │           │          │         │ plain   │ 
View definition:
 SELECT bufferid,
    relfilenode,
    reltablespace,
    reldatabase,
    relforknumber,
    relblocknumber,
    isdirty,
    usagecount,
    pinning_backends
   FROM pg_buffercache_pages() p(bufferid integer, relfilenode oid, reltablespace oid, reldatabase oid, relforknumber smallint, relblocknumber bigint, isdirty boolean, usagecount smallint, pinning_backends integer);

The reason for that error is that the function is defined to return SETOF record, so you have to specify the columns returned by the function when you call it.