Clearing global statistics

Since the pg_stat_bgwriter structure was introduced in 8.3, there's
never been any way to reset its statistics back to 0. A week of
analyzing data from that every day drove me crazy enough to finally fix
this with the attached patch. This implements the TODO item "Allow the
clearing of cluster-level statistics", based on previous discussion
ending at http://archives.postgresql.org/pgsql-hackers/2009-03/msg00920.php

Here's the patch in action:

gsmith=# select checkpoints_req,buffers_alloc from pg_stat_bgwriter;
checkpoints_req | buffers_alloc
-----------------+---------------
1 | 5

gsmith=# select pg_stat_reset_global();

gsmith=# select checkpoints_req,buffers_alloc from pg_stat_bgwriter;
checkpoints_req | buffers_alloc
-----------------+---------------
0 | 0

Patch is complete including docs, it's basically just pg_stat_reset with
a different clearing mechanism at the very end. My list of potential
questions here are:

-Not sure if this should be named pg_stat_rest_global (to match the way
these are called "global stats" in the source) or
pg_stat_reset_cluster. Picked the former for V1, not attached to that
decision at all. Might even make sense to use a name that makes it
obvious the pg_stat_bgwriter data is what's targeted.

-I create a new stats message type for this, but just reuse the same
message payload structure as pg_stats_reset rather than add a new
payload structure for no good reason. That's marked with two XXX s in
the code as a questionable design decision. I can implement that too if
there's some reason it's a good idea I don't know yet.

-I grabbed what looked like an appropriate unused OID. I'm never sure
if I did that right or not though, it may need to be renumbered.

Since this whole patch is basically a cut and paste job of code that was
already there, I don't really expect it to need much discussion beyond
these minor points; wouldn't have sent it in the middle of an active
CommitFest if that weren't the case.

--
Greg Smith 2ndQuadrant Baltimore, MD
PostgreSQL Training, Services and Support
greg(at)2ndQuadrant(dot)com www.2ndQuadrant.com