Re: PG 14 release notes, first draft

On Mon, May 10, 2021 at 3:58 PM Bruce Momjian <bruce(at)momjian(dot)us> wrote:
> OK, you are confirming what Matthias suggested. I added these two
> items, which both seem to apply only to heap pages, not index pages:

That's right -- these two relate to heap pages only.

I think that Matthias compared these two to bottom-up index deletion
because all three patches are concerned about avoiding "a permanent
solution to a temporary problem". They're conceptually similar despite
being in fairly different areas. Evidently Matthias has a similar
mental model to my own when it comes to this stuff.

Unfortunately the practical significance of the line pointer patch is
hard to demonstrate with a benchmark. I believe that it is very useful
on a sufficiently long timeline and with certain workloads because of
the behavior it avoids. As I pointed out on that other thread
recently, once you have irreversible bloat very small adverse events
will eventually add up and cause big problems. When this happens it'll
be very hard or impossible to detect, since it just looks like heap
fragmentation.

That said, it's clearly an issue with one of the TPC-C tables if you
run BenchmarkSQL for days and days (just one table, though). So there
is hard evidence that line pointer bloat could get really out of hand
in at least some tables.

--
Peter Geoghegan