Re: More aggressive vacuuming of temporary tables

Greetings,

* Andres Freund (andres@anarazel.de) wrote:
> On 2020-08-28 11:46:49 -0400, Tom Lane wrote:
> > It strikes me that when we are vacuuming a temporary table (which
> > necessarily will be one of our own session), we don't really need
> > to care what the global xmin horizon is.  If we're not inside a
> > user transaction block, then there are no tuples in the table that
> > could be in-doubt anymore.  Neither are there any snapshots in our
> > session that could see any dead tuples.  Nor do we give a fig what
> > other sessions might think of those tuples.  So we could just set
> > the xmin cutoff as aggressively as possible, which is to say
> > equal to the nextXid counter.  While vacuuming a temp table is
> > perhaps not something people do very often, I think when they do
> > do it they would like us to clean out all the dead tuples not just
> > some.
> 
> That seems like a good idea.

Agreed.

> I've been toying with a patch that introduces more smarts about when a
> row is removable, by looking more closely whether a specific row
> versions are visible (e.g. in the common case of one old snapshot and
> lots of newer rows). But that's orders of magnitude more complicated. So
> going for something as simple as this seems like a good idea.

I've wondered about this for a long time- very cool that you've found
time to actually work on a patch.  A couple of different ideas were
discussed previously about how to do that kind of a check- mind talking
about what method you're using, or perhaps just sharing that patch? :)

The potential of such an improvement is huge.

Thanks!

Stephen