Re: Table AM Interface Enhancements

On Mon, 15 Apr 2024 at 19:36, Robert Haas <robertmhaas(at)gmail(dot)com> wrote:

> On Sat, Apr 13, 2024 at 5:28 AM Alexander Korotkov <aekorotkov(at)gmail(dot)com>
> wrote:
> > Yes, I think so. Table AM API deals with TIDs and block numbers, but
> > doesn't force on what they actually mean. For example, in ZedStore
> > [1], data is stored on per-column B-trees, where TID used in table AM
> > is just a logical key of that B-trees. Similarly, blockNumber is a
> > range for B-trees.
> >
> > c6fc50cb4028 and 041b96802ef are putting to acquire_sample_rows() an
> > assumption that we are sampling physical blocks as they are stored in
> > data files. That couldn't anymore be some "logical" block numbers
> > with meaning only table AM implementation knows. That was pointed out
> > by Andres [2]. I'm not sure if ZedStore is alive, but there could be
> > other table AM implementations like this, or other implementations in
> > development, etc. Anyway, I don't feel good about narrowing the API,
> > which is there from pg12.
>
> I spent some time looking at this. I think it's valid to complain
> about the tighter coupling, but c6fc50cb4028 is there starting in v14,
> so I don't think I understand why the situation after 041b96802ef is
> materially worse than what we've had for the last few releases. I
> think it is worse in the sense that, before, you could dodge the
> problem without defining USE_PREFETCH, and now you can't, but I don't
> think we can regard nonphysical block numbers as a supported scenario
> on that basis.
>
> But maybe I'm not correctly understanding the situation?
>
Hi, Robert!

In my understanding, the downside of 041b96802ef is bringing read_stream*
things from being heap-only-related up to the level
of acquire_sample_rows() that is not supposed to be tied to heap. And
changing *_analyze_next_block() function signature to use ReadStream
explicitly in the signature.

Regards,
Pavel.