On Tue, 2009-01-20 at 11:03 +0200, Heikki Linnakangas wrote: > > - Are there some conditions where whole-table-scanning vacuum is > more > > effective than vacuums using visibility map? If so, we should > switch > > to full-scan *automatically*, without relying on user > configurations. > > Hmm, the only downside I can see is that skipping a page here and > there could defeat the OS read-ahead. Perhaps we should call > posix_fadvise(SEQUENTIAL) to compensate. Or, we could modify the logic > to only skip pages when there's at least N consecutive pages that can > be skipped.
I would rather we didn't skip any pages at all unless the gains are significant. Skipping the odd page makes no difference from a performance perspective but may have a robustness impact. "Significant gains" should take into account the size of both heap and indexes, and recognise that we still scan whole indexes in either case. -- Simon Riggs www.2ndQuadrant.com PostgreSQL Training, Services and Support -- Sent via pgsql-hackers mailing list (pgsql-hackers@postgresql.org) To make changes to your subscription: http://www.postgresql.org/mailpref/pgsql-hackers
