Re: Preventing indirection for IndexPageGetOpaque for known-size page special areas

Greetings,

* Peter Geoghegan (pg(at)bowt(dot)ie) wrote:
> On Thu, Apr 7, 2022 at 12:37 PM Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
> > On Thu, Apr 7, 2022 at 3:27 PM Peter Geoghegan <pg(at)bowt(dot)ie> wrote:
> > > I just meant that it wouldn't be reasonable to impose a fixed cost on
> > > every user, even those not using the feature. Which you said yourself.
> >
> > Unfortunately, I think there's bound to be some cost. We can avoid
> > using the space in the page for every user, but anything that makes
> > the page layout variable is going to cost some number of CPU cycles
> > someplace. We have to measure that overhead and see if it's small
> > enough that we're OK with it.
>
> I wouldn't go so far as to say that any non-zero overhead is not okay
> (that sounds really extreme). I would only say this much: wasting
> non-trivial amounts of space on every page must not happen. If the
> user opts-in to the feature then it's not just waste, so that's
> perfectly okay. (I imagine that we agree on this much already.)

Right, the *space* wouldn't ever be wasted- either the user opts-in and
the space is used for what the user asked it to be used for, or ther
user doesn't select that option and we don't reserve that space.
Robert's point was that by allowing this to happen at initdb time, there
may be some CPU cycles that end up getting spent, even if the user
didn't opt-in, that would be saved if this decision was made at compile
time. For my 2c, I'd think that would be our main concern with this,
but I also am hopeful and strongly suspect that the extra CPU cycles
aren't likely to be much and therefore would be acceptable.

A thought that's further down the line would be the question of if we
could get those CPU cycles back (and perhaps more..) by using JIT.

> > > Immediately before the special area proper (say BTOpaque), which would
> > > "logically come after" the special space under this scheme. You
> > > wouldn't have a simple constant offset into the page, but you'd have
> > > something not too far removed from such a constant. It could work as a
> > > constant with minimal context (just the AM type). Just like with
> > > Matthias' patch.
> >
> > I don't think this would work, because I don't think it would be
> > practical to always know the AM type. Think about applying an XLOG_FPI
> > record, for example.
>
> There are already some pretty shaky heuristics that are used by tools
> like pg_filedump for this exact purpose. But they work! And they're
> even supported by Postgres, quasi-officially -- grep for "pg_filedump"
> to see what I mean.

Shaky heuristics feels like something appropriate for pg_filedump.. less
so for things like TDE.

> There are numerous reasons why we might want to put that on a formal
> footing, so that we can reliably detect the AM type starting from only
> a page image. I suspect that you're going to end up needing to account
> for this index AMs anyway, so going this way isn't necessarily making
> your life all that much harder. (I am not really sure about that,
> though, since I don't have enough background information.)

This seems like it'd be pretty difficult given that AMs can be loaded as
extensions..? Also seems like a much bigger change and I'm still not
sure what the advantage is, at least in terms of what this particular
patch is doing.

Thanks,

Stephen