Re: Clarify vacuum verbose message

On Tue, Dec 15, 2015 at 9:19 PM, Jim Nasby <Jim(dot)Nasby(at)bluetreble(dot)com> wrote:
> VACUUM VERBOSE spits out two different messages for the heap, one of which
> is rather confusing:
>
> INFO: "trades": removed 625664 row versions in 20967 pages
> ...
> INFO: "trades": found 3282 removable, 56891627 nonremovable row versions in
> 1986034 out of 1986034 pages
>
> After discussion with RhodiumToad I think I now understand how this can
> happen:
>
> 20:00 < RhodiumToad> the LP_DEAD slot is where the index entries for the
> deleted row point to, so that has to stay
> 20:01 < RhodiumToad> so for example, if you delete a lot of rows, then try
> and do a lot of updates (which will hint the
> pages as needing pruning),
> 20:01 < RhodiumToad> then do more updates or a seqscan (to let prune look at
> the pages),
> 20:02 < RhodiumToad> then do a vacuum, the vacuum will see a lot of LP_DEAD
> slots to remove index entries for, but not
> actual tuples
>
> This example is from a table that was VACUUM FULL'd this weekend and had a
> nightly batch process run last night. That process INSERTs a bunch of rows
> and then does a bunch of UPDATEs on different subsets of those rows. I don't
> believe there would have been a large amount of deletes; I'll check with
> them tomorrow.
>
> IMHO we need to change the messages so they are explicit about line pointers
> vs actual tuples. Trying to obfuscate that just leads to confusion.
> heap_page_prune needs to report only non-rootlp tuples that were pruned.
> (None of the other callers care about the return value.)

Yeah, I've had the the thought before that this reporting could be
more clear. I think it never really got revised when 8.3 invented
HOT. It might be about time for that.

--
Robert Haas
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company