Autovacuum and OldestXmin

I notice that slony records the oldestxmin that was running when it last
ran a VACUUM on its tables. This allows slony to avoid running a VACUUM
when it would be clearly pointless to do so.

AFAICS autovacuum does not do this, or did I miss that?

It seems easy to add (another, groan) column onto pg_stat_user_tables to
record the oldestxmin when it was last vacuumed. (last_autovacuum_xmin)

That will avoid pointless VACUUMs for all users (in 8.4).

Strangely HOT does this at the page level to avoid useless work, yet
stranger still VACUUM doesn't evaluate PageIsPrunable() at all and
always scans each page regardless.

Why isn't VACUUM optimised the same way HOT is?
Why doesn't VACUUM continue onto the next block when !PageIsPrunable().
Nothing is documented though it seems "obvious" that it should.

Perhaps an integration oversight?

[Also there is a comment saying "this is a bug" in autovacuum.c
Are we thinking to go production with that phrase in the code?]

--
Simon Riggs
2ndQuadrant http://www.2ndQuadrant.com