dump cache summary

As part of attempting to gain an understanding of how Postgres works, I
wanted to see if I could display a summary of what relations were using
pages in the cache.

Having done that, I was all set to trash the code when I wondered if it
might be useful in its own right...

Here is a sample of the output after a pgbench run with shared_buffers=500:

bench=>dumpcache;
                           CACHE DUMP
-----------------------------------------------------------------
 pg_type_oid_index                                             2
 pg_proc                                                       1
 history                                                       3
 pg_class_relname_nsp_index                                    5
 pg_statistic_relid_att_index                                  2
 tellers_pkey                                                  9
 <invalid oid>                                                 4
 accounts                                                    205
 pg_am                                                         1
 pg_statistic                                                  1
 branches                                                      2
 accounts_pkey                                               215
 pg_amproc_opc_procnum_index                                   2
 branches_pkey                                                 9
 pg_operator_oid_index                                         3
 tellers                                                       4
 pg_class_oid_index                                            2
 pg_type                                                       2
 pg_index_indexrelid_index                                     2
 pg_trigger_tgrelid_tgname_index                               2
 pg_amop_opr_opc_index                                         2
 pg_index_indrelid_index                                       2
 pg_class                                                      2
 pg_trigger                                                    1
 pg_proc_oid_index                                             1
 pg_amop_opc_strategy_index                                    2
 pg_attribute_relid_attnum_index                               4
 pg_amop                                                       1
 pg_amproc                                                     1
 pg_index                                                      2
 pg_operator                                                   3
 pg_attribute                                                  3
(32 rows)

bench=>

Does this seem like a useful thing to be able to display ?

I implemented this by adding a command (as I wanted to know how this was
done), but I suspect it would make more sense to use a function a bit
like the pg_stat* collection.

I have included (most of) the code I used, so that interested parties
can show me what I have done wrong :-)

(Some things I wondered about were : should I be locking buffers before
peeking at the corresponding descriptors?, does it make sense to call
RelationIdGetRelation on a relNode?.... )

regards

Mark