Re: brin index vacuum versus transaction snapshots

Robert Haas wrote:
> On Fri, Jul 31, 2015 at 3:45 PM, Alvaro Herrera
> <alvherre@2ndquadrant.com> wrote:

> > I think the only way to close this hole is to have summarize_range()
> > sleep until all open snapshots are gone after inserting the placeholder
> > tuple and before acquiring the snapshot, similarly to how CREATE INDEX
> > CONCURRENTLY does it.
> 
> That's gonna be really slow, though, right?  Even if you rework things
> so that vacuum inserts all the placeholder tuples first, then waits,
> then does all the summarization, that could easily turn a vacuum that
> would have finished in a second into one that instead takes multiple
> hours.  During that time an AV worker is pinned down, and all sorts of
> badness can ensue.

Yeah, it is bad and I was concerned about that too.  Thankfully I found
another way to solve it, which is to forgo usage of MVCC here and
instead use SnapshotAny.  There's already a mode in
IndexBuildHeapRangeScan that uses SnapshotAny, but it needs some tweaks
to do what we need.  I'm going to propose a patch along those lines
shortly.

> Maybe I'm misunderstanding, but this whole thing seems like a pretty
> serious problem for BRIN.  :-(

With this new approach it shouldn't be.

-- 
Álvaro Herrera                http://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services