Re: Add index scan progress to pg_stat_progress_vacuum

> On Wed, Feb 23, 2022 at 10:02 AM Imseih (AWS), Sami <simseih(at)amazon(dot)com> wrote:
>> If the failsafe kicks in midway through a vacuum, the number indexes_total will not be reset to 0. If INDEX_CLEANUP is turned off, then the value will be 0 at the start of the vacuum.
>
> The way that this works with num_index_scans is that we "round up"
> when there has been non-zero work in lazy_vacuum_all_indexes(), but
> not if the precheck in lazy_vacuum_all_indexes() fails. That seems
> like a good model to generalize from here. Note that this makes
> INDEX_CLEANUP=off affect num_index_scans in much the same way as a
> VACUUM where the failsafe kicks in very early, during the initial heap
> pass. That is, if the failsafe kicks in before we reach lazy_vacuum()
> for the first time (which is not unlikely), or even in the
> lazy_vacuum_all_indexes() precheck, then num_index_scans will remain
> at 0, just like INDEX_CLEANUP=off.
>
> The actual failsafe WARNING shows num_index_scans, possibly before it
> gets incremented one last time (by "rounding up"). So it's reasonably
> clear how this all works from that context (assuming that the
> autovacuum logging stuff, which reports num_index_scans, outputs a
> report for a table where the failsafe kicked in).

> I am confused. If failsafe kicks in during the middle of a vacuum, I
> (perhaps naively) would expect indexes_total and indexes_processed to go to
> zero, and I'd expect to no longer see the "vacuuming indexes" and "cleaning
> up indexes" phases. Otherwise, how would I know that we are now skipping
> indexes? Of course, you won't have any historical context about the index
> work done before failsafe kicked in, but IMO it is misleading to still
> include it in the progress view.

Failsafe occurring in the middle of a vacuum and resetting "indexes_total" to 0 will be misleading. I am thinking that it is a better idea to expose only one column "indexes_remaining".

If index_cleanup is set to OFF, the values of indexes_remaining will be 0 at the start of the vacuum.
If failsafe kicks in during a vacuum in-progress, "indexes_remaining" will be calculated to 0.

This approach will provide a progress based on how many indexes remaining with no ambiguity.

--
Sami Imseih
Amazon Web Services