Re: Show various offset arrays for heap WAL records

Hi,

On 2023-01-09 19:59:42 -0800, Peter Geoghegan wrote:
> On Mon, Jan 9, 2023 at 1:58 PM Andres Freund <andres(at)anarazel(dot)de> wrote:
> > A couple times when investigating data corruption issues, the last time just
> > yesterday in [1], I needed to see the offsets affected by PRUNE and VACUUM
> > records. As that's probably not just me, I think we should make that change
> > in-tree.
>
> I remember how useful this was when we were investigating that early
> bug in 14, that turned out to be in parallel VACUUM. So I'm all in
> favor of it.

Cool.

> > The attached patch adds details to XLOG_HEAP2_PRUNE, XLOG_HEAP2_VACUUM,
> > XLOG_HEAP2_FREEZE_PAGE.
>
> I'm bound to end up doing the same in index access methods. Might make
> sense for the utility routines to live somewhere more centralized, at
> least when code reuse is likely. Practically every index AM has WAL
> records that include a sorted page offset number array, just like
> these ones. It's a very standard thing, obviously.

Hm, there doesn't seem to be a great location for them today. I guess we could
add something like src/include/access/rmgrdesc_utils.h? And put the
implementation in src/backend/access/rmgrdesc/rmgrdesc_utils.c? I first was
thinking of just rmgrdesc.[ch], but custom rmgrs added
src/bin/pg_waldump/rmgrdesc.[ch] ...

> > I chose to include infomask[2] for the different freeze plans mainly because
> > it looks odd to see different plans without a visible reason. But I'm not sure
> > that's the right choice.
>
> I don't think that it is particularly necessary to do so in order for
> the output to make sense -- pg_waldump is inherently a tool for
> experts. What it comes down to for me is whether or not this
> information is sufficiently useful to display, and/or can be (or needs
> to be) controlled via some kind of verbosity knob.

It seemed useful enough to me, but I likely also stare more at this stuff than
most. Compared to the list of offsets it's not that much content.

> How hard would it be to invent a general mechanism to control the verbosity
> of what we'll show for each WAL record?

Nontrivial, I'm afraid. We don't pass any relevant parameters to rm_desc:
void (*rm_desc) (StringInfo buf, XLogReaderState *record);

so we'd need to patch all of them. That might be worth doing at some point,
but I don't want to tackle it right now.

Greetings,

Andres Freund