Add usage counts to pg_buffercache

This patch adds the usage count statistic to the information available in
contrib/pgbuffercache. Earlier this month a discussion about my first
attempt to instrument the background writer had Tom asking for details
about the usage histogram I was seeing, and this patch proved to be the
easiest way I found to take a look at that.

In situations where one is trying to optimize the background writer, it's
very hard to adjust how much to rely on the LRU writer versus the one that
writes everything unless you know whether your dirty buffers are typically
used heavily (like index blocks) or not (like new INSERT data). Some
statistics about the usage counts in your buffer cache are extremely
helpful in making that decision.

I'll even pass along an ugly but fun query that utilizes this. The
following will give you a summary of your buffer cache broken into 32
sections. Each line shows the average usage count of that section, as a
positive number if most buffers dirty and a negative one if most are
clean. If you refresh this frequently enough, you can actually watch
things like how checkpoints move through the buffer cache:

SELECT current_timestamp,
-- Split into 32 bins of data
round(bufferid / (cast((select setting from pg_settings where
name='shared_buffers') as int) / (32 - 1.0)))
as section, round(
-- Average usage count, capped at 5
case when avg(usagecount)>5 then 5 else avg(usagecount) end *
-- -1 when the majority are clean records, 1 when most are dirty
(case when sum(case when isdirty then 1 else -1 end)>0 then 1 else -1
end)) as color_intensity
FROM pg_buffercache GROUP BY
round(bufferid / (cast((select setting from pg_settings where
name='shared_buffers') as int) / (32 - 1.0)));

The 32 can be changed to anything, that's just what fits on my screen.
The main idea of the above is that if you dump all this to a file
regularly, it's possible to produce a graph of it showing how the cache
has changed over time by assigning a different color intensity based on
the usage count--at a massive cost in overhead, of course. I'll be
passing along all that code once I get it ready for other people to use.

--
* Greg Smith gsmith(at)gregsmith(dot)com http://www.gregsmith.com Baltimore, MD