Re: Add pg_stat_autovacuum_priority

I didn't read any of the thread, but I looked at the patches.

0001:

+ * force_scores set to true forces the computation of a score. This is useful for
+ * tools that wish to inspect scores outside of the do_vacuum() path.

I'm of two minds about this new function parameter.  On one hand, I see the
utility of forcing score calculations even when autovacuum is disabled.  On
the other hand, when autovacuum is disabled, the scores are actually 0.0,
and it's probably a good idea to report exactly what autovacuum workers
see.  I also see that we're not forcing the computation of the (M)XID
scores.  Is that intentional?

I wonder if we can rework this function to always calculate the scores,
even if autovacuum is disabled or !force_vacuum.  This way, both paths are
doing the exact same thing and reporting the same scores.  We might still
want an extra parameter to avoid DEBUG3 in the system view path, but that
seems like a reasonable difference between the two.

0002:

Seems okay to me.

0003:

+     <row>
+      <entry role="catalog_table_entry"><para role="column_definition">
+       <structfield>vacuum_dead_score</structfield> <type>double precision</type>
+      </para>
+      <para>
+       Score component based on the estimated number of dead tuples
+       needing removal by vacuum.
+      </para></entry>
+     </row>

I think we should make sure the column names align with the names given to
the new parameters [0] and the new "Autovacuum Prioritization" section in
the docs [1].

[0] https://www.postgresql.org/docs/devel/runtime-config-vacuum.html#GUC-AUTOVACUUM-VACUUM-SCORE-WEIGHT
[1] https://www.postgresql.org/docs/devel/routine-vacuuming.html#AUTOVACUUM-PRIORITY

-- 
nathan