Re: Update comment for README.HOT

Tom Lane wrote:
> Bruce Momjian <bruce@momjian.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@momjian.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
  ------