Re: Update comment for README.HOT

Applied.

---------------------------------------------------------------------------

Bruce Momjian wrote:
> Tom Lane wrote:
> > Bruce Momjian <bruce(at)momjian(dot)us> writes:
> > > + This means that UPDATE, DELETE, and SELECT can trigger space
> > > + reclamation, while INSERT ... VALUES cannot because it does not retrieve
> > > + a row.
> >
> > I don't believe that's correct. It might have happened to work that way
> > for you in a particular test. It's certainly not something I'd document
> > as being intended long-term behavior.
>
> Well, I would like to document something about this because I was
> surprised that when INSERT did not trigger a cleanup. I realize we
> might change the behavior but then we would update the file too,
> hopefully.
>
> How is the attached version using "often"? I also clarified it is < 10%
> free.
>
> I found this while doing tests for a new MVCC talk I will be delivering
> at PG West.
>
> --
> Bruce Momjian <bruce(at)momjian(dot)us> http://momjian.us
> EnterpriseDB http://enterprisedb.com
>
> + It's impossible for everything to be true. +

> Index: src/backend/access/heap/README.HOT
> ===================================================================
> RCS file: /cvsroot/pgsql/src/backend/access/heap/README.HOT,v
> retrieving revision 1.6
> diff -c -c -r1.6 README.HOT
> *** src/backend/access/heap/README.HOT 23 Apr 2010 23:21:44 -0000 1.6
> --- src/backend/access/heap/README.HOT 17 Sep 2010 21:21:56 -0000
> ***************
> *** 246,251 ****
> --- 246,257 ----
> is arbitrarily capped at MaxHeapTuplesPerPage (the most tuples that
> could fit without HOT pruning).
>
> + Effectively, space reclamation happens during tuple retrieval when the
> + page is nearly full (<10% free) and a buffer cleanup lock can be
> + acquired. This means that UPDATE, DELETE, and SELECT can trigger space
> + reclamation, but often not during INSERT ... VALUES because it does
> + not retrieve a row.
> +
>
> VACUUM
> ------

>
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--
Bruce Momjian <bruce(at)momjian(dot)us> http://momjian.us
EnterpriseDB http://enterprisedb.com

+ It's impossible for everything to be true. +