Re: PG 14 release notes, first draft

On Tue, May 18, 2021 at 06:28:49PM +0900, Masahiko Sawada wrote:
> On Mon, May 10, 2021 at 3:03 PM Bruce Momjian  wrote:
> >
> > I have committed the first draft of the PG 14 release notes.  You can
> > see the most current  build of them here:
> >
> >         https://momjian.us/pgsql_docs/release-14.html
> >
> 
> I think we need to mention in the release note that
> vacuum_cleanup_index_scale_factor GUC parameter has been removed and
> vacuum_cleanup_index_scale_factor storage parameter has been
> deprecated (please refer to commit 9f3665fb and effdd3f3b63).

Looking at the full commit message:

	commit 9f3665fbfc
	Author: Peter Geoghegan 
	Date:   Wed Mar 10 16:27:01 2021 -0800
	
	    Don't consider newly inserted tuples in nbtree VACUUM.
	
	    Remove the entire idea of "stale stats" within nbtree VACUUM (stop
	    caring about stats involving the number of inserted tuples).  Also
	    remove the vacuum_cleanup_index_scale_factor GUC/param on the master
	    branch (though just disable them on postgres 13).
	
	    The vacuum_cleanup_index_scale_factor/stats interface made the nbtree AM
	    partially responsible for deciding when pg_class.reltuples stats needed
	    to be updated.  This seems contrary to the spirit of the index AM API,
	    though -- it is not actually necessary for an index AM's bulk delete and
	    cleanup callbacks to provide accurate stats when it happens to be
	    inconvenient.  The core code owns that.  (Index AMs have the authority
	    to perform or not perform certain kinds of deferred cleanup based on
	    their own considerations, such as page deletion and recycling, but that
	    has little to do with pg_class.reltuples/num_index_tuples.)
	
	    This issue was fairly harmless until the introduction of the
	    autovacuum_vacuum_insert_threshold feature by commit b07642db, which had
	    an undesirable interaction with the vacuum_cleanup_index_scale_factor
	    mechanism: it made insert-driven autovacuums perform full index scans,
	    even though there is no real benefit to doing so.  This has been tied to
	    a regression with an append-only insert benchmark [1].
	
	    Also have remaining cases that perform a full scan of an index during a
	    cleanup-only nbtree VACUUM indicate that the final tuple count is only
	    an estimate.  This prevents vacuumlazy.c from setting the index's
	    pg_class.reltuples in those cases (it will now only update pg_class when
	    vacuumlazy.c had TIDs for nbtree to bulk delete).  This arguably fixes
	    an oversight in deduplication-related bugfix commit 48e12913.
	
	    [1] https://smalldatum.blogspot.com/2021/01/insert-benchmark-postgres-is-still.html
	
	    Author: Peter Geoghegan 
	    Reviewed-By: Masahiko Sawada 
	    Discussion: https://postgr.es/m/CAD21AoA4WHthN5uU6+WScZ7+J_RcEjmcuH94qcoUPuB42ShXzg@mail.gmail.com
-->	    Backpatch: 13-, where autovacuum_vacuum_insert_threshold was added.

This was backpatched into PG 13.3, which was released last week:

	    
	
	     
	      Disable the vacuum_cleanup_index_scale_factor
	      parameter and storage option (Peter Geoghegan)
	     
	
	     
	      The notion of tracking stale index statistics proved
	      to interact badly with
	      the autovacuum_vacuum_insert_threshold parameter,
	      resulting in unnecessary full-index scans and consequent degradation
	      of autovacuum performance.  The latter mechanism seems superior, so
	      remove the stale-statistics logic.  The control parameter for that,
	      vacuum_cleanup_index_scale_factor, will be
	      removed entirely in v14.  In v13, it remains present to avoid
	      breaking existing configuration files, but it no longer does
	      anything.
	     
	    

Therefore, it didn't show up in my src/tools/git_changelog output, and I
did not include it.

-- 
  Bruce Momjian          https://momjian.us
  EDB                                      https://enterprisedb.com

  If only the physical world exists, free will is an illusion.