Session WAL activity

Hi hackers,

One of our customers complains about that some sessions generates "too
much WAL records".
Certainly WAL activity doesn't indicate a problem itself: huge workload
cause huge WAL activity.
But them are trying to understand which clients produces so much
database changes and complain that there is
no way to get such information in Postgres. For example in Oracle this
problems can be solved in this way:

http://www.dba-oracle.com/t_find_session_generating_high_redo.htm

Unfortunately there is actually no simple and accurate way to calculate
amount of WAL produced by the particular session.
It is possible to parse WAL (for example using pg_waldump), then using
XID->pid mapping accumulate size of transactions produced by each backend.
But this is very inconvenient and not DBA friendly approach.

I have implemented small patch which collects such statistic.
I have added walWritten  field to PGPROC and increment it in
CopyXLogRecordToWAL.
It is possible to inspect this field using pg_stat_get_wal_activity(pid)
function and also I have added
pg_stat_wal_activity which just adds  wal_written to standard
pg_activity view:

postgres=# select pid, backend_type, wal_written from pg_stat_wal_activity ;
pid | backend_type | wal_written
------+------------------------------+-------------
4405 | autovacuum launcher | 0
4407 | logical replication launcher | 0
4750 | client backend | 86195
4403 | background writer | 204
4402 | checkpointer | 328
4404 | walwriter | 0
(6 rows)

I wonder if such extra statistic about session WAL activity is
considered to be useful?
The only problem with this approach from my point of view is adding 8
bytes to PGPROC.
But there are already so many fields in this structure
(sizeof(PGPROC)=816), that adding yet another 8 bytes should not be
noticeable.

Comments are welcome.

--
Konstantin Knizhnik
Postgres Professional: http://www.postgrespro.com
The Russian Postgres Company