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