Re: [HACKERS] GSoC on WAL-logging hash indexes

On Wed, Apr 30, 2014 at 11:02 AM, Peter Geoghegan <pg(at)heroku(dot)com> wrote:

> On Wed, Apr 30, 2014 at 10:11 AM, Robert Haas <robertmhaas(at)gmail(dot)com>
> wrote:
> > I thought the theoretical advantage of hash indexes wasn't that they
> > were smaller but that you avoided a central contention point (the
> > btree root).
>
> The B-Tree root isn't really a central contention point at all. The
> locking/latching protocol that nbtree uses is remarkably
> concurrency-friendly. In the real world, there is pretty much no
> exclusive locking of the root page's buffer.
>

I've seen the simple pinning and unpinning of the root page (or the fast
root, whatever the first page we bother to pin on a regular basis is
called) be a point of contention. When one index dominates the entire
system workload, that one page also drives contention on the spin lock that
protects the lwlock that share-protects whichever buffer mapping partition
happens to contain it.

Cheers,

Jeff