Re: [PATCH] Full support for index LP_DEAD hint bits on standby

Michail Nikolaev <michail(dot)nikolaev(at)gmail(dot)com> wrote:

> Hello, Antonin.
>
> > I'm trying to review the patch, but not sure if I understand this problem,
> > please see my comment below.
>
> Thanks a lot for your attention. It is strongly recommended to look at
> version N3 (1) because it is a much more elegant, easy, and reliable
> solution :) But the minRecoveryPoint-related issue affects it anyway.

Indeed I'm reviewing (1), but I wanted to discuss this particular question in
context, so I replied here.

> > Why doesn't minRecoveryPoint get updated to 20? IMO that should happen by
> > replaying the commit record. And if the standby happens to crash before the
> > commit record could be replayed, no query should see the deletion and thus no
> > hint bit should be set in the index.
>
> minRecoveryPoint is not affected by replaying the commit record in
> most cases. It is updated in a lazy way, something like this:
> minRecoveryPoint = max LSN of flushed page. Version 3 of a patch
> contains a code_optional.patch to move minRecoveryPoint more
> aggressively to get additional performance on standby (based on
> Peter’s answer in (2).

> So, “minRecoveryPoint will go here” is not because of “STANDBY INSERTS
> NEW ROW IN INDEX” it is just a random event.
> Michail.

Sorry, I missed the fact that your example can be executed inside BEGIN - END
block, in which case minRecoveryPoint won't advance after each command.

I'll continue my review by replying to (1)

> [1]: https://www.postgresql.org/message-id/CANtu0ohHu1r1xQfTzEJuxeaOMYncG7xRxUQWdH%3DcMXZSf%2Bnzvg%40mail.gmail.com
> [2]: https://www.postgresql.org/message-id/CAH2-WzkSUcuFukhJdSxHFgtL6zEQgNhgOzNBiTbP_4u%3Dk6igAg%40mail.gmail.com

> (“Also, btw, do you know any reason to keep minRecoveryPoint at a low
> value?”)

I'm not an expert in this area (I'm reviewing this patch also to learn more
about recovery and replication), but after a breif research I think that
postgres tries not to update the control file too frequently, see comments in
UpdateMinRecoveryPoint(). I don't know if what you do in code_optional.patch
would be a problem. Actually I think that a commit record should be replayed
more often than XLOG_RUNNING_XACTS, shouldn't it?

--
Antonin Houska
Web: https://www.cybertec-postgresql.com