Re: [DOCS] HOT - correct claim about indexes not referencing old line pointers

On Wed, Oct 4, 2023 at 9:18 AM Robert Haas <robertmhaas(at)gmail(dot)com> wrote:
>
> On Tue, Oct 3, 2023 at 3:35 PM James Coleman <jtc331(at)gmail(dot)com> wrote:
> > I like your changes. Reading through this several times, and noting
> > Peter's comments about pruning being more than just HOT, I'm thinking
> > that rather than a simple fixup for this one paragraph what we
> > actually want is to split out the concept of page pruning into its own
> > section of the storage docs. Attached is a patch that does that,
> > incorporating much of your language about LP_REDIRECT, along with
> > LP_DEAD so that readers know this affects more than just heap-only
> > tuple workloads.
>
> I considered this kind of approach, but I don't think it's better. I
> think the point of this documentation is to give people a general idea
> of how HOT works, not all the technical details. If we start getting
> into the nitty gritty, we're going to have to explain a lot more
> stuff, which is going to detract from the actual purpose of this
> documentation. For example, your patch talks about LP_REDIRECT and
> LP_DEAD, which are not referenced in any other part of the
> documentation. It also uses the term line pointer instead of page item
> identifier without explaining that those are basically the same thing.
> Obviously those kinds of things can be fixed, but in my opinion, your
> version doesn't really add much information that is likely to be
> useful to a reader of this section, while at the same time it does add
> some things that might be confusing. If we wanted to have a more
> technical discussion of all of this somewhere in the documentation, we
> could, but that would be quite a bit more work to write and review,
> and it would probably duplicate some of what we've already got in
> README.HOT.
>
> I suggest that we should be looking for a patch that tries to correct
> the wrong stuff in the present wording while adding the minimum
> possible amount of text.

There's one primary thing I think this approach adds (I don't mean the
patch I proposed is the only way to address this concern): the fact
that pages get cleaned up outside of the HOT optimization. We could
add a short paragraph about that on the HOT page, but that seems a bit
confusing.

Are you thinking we should simply elide the fact that there is pruning
that happens outside of HOT? Or add that information onto the HOT
page, even though it doesn't directly fit?

Regards,
James