Jeff Janes wrote:
> It is only a page read if you have to read the page. It would seem optimal
> to have bgwriter adventitiously set hint bits and vm bits, because that is
> the last point at which the page can be changed without risking that it be
> written out twice. At that point, it has been given the maximum amount of
> time it can be given for the interested transactions to have committed and
> to have aged past the xmin horizon. I seem to recall that the main problem
> with that, though, is that you must be attached to a database in order to
> determine visibility, and bgwriter is not attached to a database.
Regarding tuple hint bits, I couldn't find any such limitation in
SetHintBits, other than in MarkBufferDirtyHint there being some code
that would cause trouble: it accesses MyPgXact, which bgwriter would
obviously not have. Maybe worth some experimentation ...
I'm not sure about vm bits, though. That's a whole different topic.
--
Álvaro Herrera http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Training & Services
