Re: VACUUM's ancillary tasks

On Mon, Aug 29, 2016 at 1:26 PM, Vik Fearing <vik(at)2ndquadrant(dot)fr> wrote:
> While doing this, I noticed that ALTER TABLE should also re-analyze the
> table for obvious reasons, so I have that one set
> "changes_since_analyze" to the number of rows rewritten as well.

I noticed that ATExecAlterColumnType does this:

/*

* Drop any pg_statistic entry for the column, since it's now wrong
type
*/

RemoveStatistics(RelationGetRelid(rel), attnum);

What if there is no rewrite, because the type is binary compatible
(ATColumnChangeRequiresRewrite returns false)? Then I think your patch
won't attract an autovacuum daemon to ANALYZE the table and produce new
statistics, because it won't touch changes_since_analyze. I wonder if we
should simply keep the stats if no rewrite is required. Aren't the
statistical properties of binary-compatible types also compatible?
Alternatively, we should consider bumping changes_since_analyze in this
case too, if you're going to do it in the rewrite case.

--
Thomas Munro
http://www.enterprisedb.com