Bulk inserts and usage_count

While testing the buffer ring patch, I noticed that bulk inserts with 
both INSERT and COPY pin and unpin the buffer they insert to for every 
tuple. That means that the usage_count of all those buffers are bumped 
up to 5. That's gotta be bad if you try to run a COPY concurrently with 
other activity. It also affects tables like TPC-C order_line where a 
tuples are always inserted and updated in groups.

To demonstrate:

postgres=# truncate foo; TRUNCATE TABLE
postgres=# COPY foo FROM '/tmp/foo.data'; COPY 1000
postgres=# SELECT c.relname, bufferid, relblocknumber, isdirty, 
usagecount FROM pg_buffercache bc, pg_class c WHERE c.relfilenode = 
bc.relfilenode and c.relname='foo'; relname | bufferid | relblocknumber | isdirty | usagecount
---------+----------+----------------+---------+------------ foo     |   105078 |              4 | f       |          5
foo    |   105079 |              3 | f       |          5 foo     |   105080 |              2 | f       |          5
foo    |   105081 |              1 | f       |          5 foo     |   105082 |              0 | f       |          5
 
(5 rows)

A fix for COPY will fall naturally out of the buffer ring patch, but not 
for INSERT.

A more general fix would be to somehow keep the last insertion page 
pinned across calls to heap_insert.

--   Heikki Linnakangas  EnterpriseDB   http://www.enterprisedb.com