Buffer statistics for pg_stat_statements

We have infrastructure to count numbers buffer access in 8.5 Alpha 3.
I'd like to add per-query buffer usage into contrib/pg_stat_statements.

The pg_stat_statements view will have the same contents with
struct BufferUsage. Fields named shared_blks_{hit|read|written},
local_blks_{hit|read|written}, and temp_blks_{read|written}
will be added to the view.

We can determine slow queries not only based on durations but also
based on I/O or memory access count. Also, queries with non-zero
temp_blks_read means DBA need to consider increasing work_mem. Those
information would be useful to find where the server's bottleneck is.

Additional management costs cannot be avoided, but I think it should be
not so high because we accumulate buffer usage only once per query,
while EXPLAIN BUFFERS is slow because we need per-tuple calculation.

I'll submit this pg_stat_statements enhancement to the next commit fest.
Comments welcome.

Regards,
---
Takahiro Itagaki
NTT Open Source Software Center