monitoring sort activities

Hi folks,

I've created a patch to get following TODO item.

> * %Add ability to monitor the use of temporary sort files

This patch provides one system view (pg_stat_sorts) and
one utility function (pg_stat_get_prev_sort_size).

> snaga=# SELECT * from pg_stat_sorts;
> heap_all | index_all | datum_all | heap_tape | datum_tape | max_size
> ----------+-----------+-----------+-----------+------------+----------
> 4 | 6 | 0 | 4 | 0 | 26400000
> (1 row)
>
> snaga=# select aid from accounts where aid % 2 = 1 order by filler
desc limit 10;
>
> [...snip...]
>
> (10 rows)
>
> snaga=# SELECT pg_stat_get_prev_sort_size();
> pg_stat_get_prev_sort_size
> ----------------------------
> 13200000
> (1 row)

Yeah, I don't forget previous discussion with Tom. We also have to
monitor temp files, but I think these view and function will benefit
DBAs on performance tuning.

Any comments or suggestion?
--
NAGAYASU Satoshi <nagayasus(at)nttdata(dot)co(dot)jp>