Robert Haas wrote:
> On Fri, Apr 24, 2015 at 3:04 PM, Alvaro Herrera
> <alvherre@2ndquadrant.com> wrote:
> > I think what we need here is something that does heap_update to tuples
> > at the end of the table, moving them to earlier pages; then wait for old
> > snapshots to die (the infrastructure for which we have now, thanks to
> > CREATE INDEX CONCURRENTLY); then truncate the empty pages. Of course,
> > there are lots of details to resolve. It doesn't really matter that
> > this runs for long: a process doing this for hours might be better than
> > AccessExclusiveLock on the table for a much shorter period.
>
> Why do you need to do anything other than update the tuples and let
> autovacuum clean up the mess?
Sure, that's one option. I think autovac's current approach is too
heavyweight: it always has to scan the whole relation and all the
indexes. It might be more convenient to do something more
fine-grained; for instance, maybe instead of scanning the whole
relation, start from the end of the relation walking backwards and stop
once the first page containing a live or recently-dead tuple is found.
Perhaps, while scanning the indexes you know that all CTIDs with pages
higher than some threshold value are gone; you can remove them without
scanning the heap at all perhaps.
--
Álvaro Herrera http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
