Tom Lane wrote:
> Alvaro Herrera <alvherre@alvh.no-ip.org> writes:
> > 2. when summarization is requested on the partial range at the end of a
> > table, we acquire extension lock on the rel, then compute relation size
> > and run summarization with the lock held. This guarantees that we don't
> > miss any pages. This is bad for concurrency though, so it's only done
> > in that specific scenario.
>
> Hm, I wonder how this will play with the active proposals around
> reimplementing relation extension locks. All that work seems to be
> assuming that the extension lock is only held for a short time and
> nothing much beyond physical extension is done while holding it.
> I'm afraid that you may be introducing a risk of e.g. deadlocks
> if you do this.
Ouch ... yeah, that could be a problem.
Another idea I had was to just insert the placeholder tuple while
holding the extension lock, then release the lock while the
summarization is done. It would be a bit of a break of the current
separation of concerns, but I'm not convinced that the current setup is
perfect, so maybe that's okay.
> If VACUUM and brin_summarize_new_values both ignore the partial
> range, then what else would request this? Can't we just decree
> that we don't summarize the partial range, period?
brin_summarize_range() can do it.
--
Álvaro Herrera https://www.2ndQuadrant.com/
PostgreSQL Development, 24x7 Support, Remote DBA, Training & Services
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