Re: New vacuum option to do only freezing

On Tue, Mar 5, 2019 at 11:29 PM Masahiko Sawada <sawada.mshk@gmail.com> wrote:
> Attached updated patch incorporated all of comments. Also I've added
> new reloption vacuum_index_cleanup as per discussion on the "reloption
> to prevent VACUUM from truncating empty pages at the end of relation"
> thread. Autovacuums also can skip index cleanup when the reloption is
> set to false. Since the setting this to false might lead some problems
> I've made autovacuums report the number of dead tuples and dead
> itemids we left.

It seems to me that the disable_index_cleanup should be renamed
index_cleanup and the default should be changed to true, for
consistency with the reloption (and, perhaps, other patches).

- num_tuples = live_tuples = tups_vacuumed = nkeep = nunused = 0;
+ num_tuples = live_tuples = tups_vacuumed  = nkeep = nunused =
+ nleft_dead_itemids = nleft_dead_tuples = 0;

I would suggest leaving the existing line alone (and not adding an
extra space to it as the patch does now) and just adding a second
initialization on the next line as a separate statement. a = b = c = d
= e = 0 isn't such great coding style that we should stick to it
rigorously even when it ends up having to be broken across lines.

+ /* Index vacuum must be enabled in two-pass vacuum */
+ Assert(!skip_index_vacuum);

I am a big believer in naming consistency.  Please, let's use the same
name everywhere!  If it's going to be index_cleanup, then call the
reloption vacuum_index_cleanup, and call the option index_cleanup, and
call the variable index_cleanup.  Picking a different subset of
cleanup, index, vacuum, skip, and disable for each new name makes it
harder to understand.

- * If there are no indexes then we can vacuum the page right now
- * instead of doing a second scan.
+ * If there are no indexes or index vacuum is disabled we can
+ * vacuum the page right now instead of doing a second scan.

This comment is wrong.  That wouldn't be safe.  And that's probably
why it's not what the code does.

- /* If no indexes, make log report that lazy_vacuum_heap would've made */
+ /*
+ * If no index or index vacuum is disabled, make log report that
+ * lazy_vacuum_heap would've make.
+ */
  if (vacuumed_pages)

Hmm, does this really do what the comment claims?  It looks to me like
we only increment vacuumed_pages when we call lazy_vacuum_page(), and
we (correctly) don't do that when index cleanup is disabled, but then
here this claims that if (vacuumed_pages) will be true in that case.

I wonder if it would be cleaner to rename vacrelstate->hasindex to
'useindex' and set it to false if there are no indexes or index
cleanup is disabled.  But that might actually be worse, not sure.

-- 
Robert Haas
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