Re: BitmapHeapScan streaming read user and prelim refactoring

On Tue, Feb 13, 2024 at 11:34 PM Mark Dilger
<mark(dot)dilger(at)enterprisedb(dot)com> wrote:
>
> > On Feb 13, 2024, at 3:11 PM, Melanie Plageman <melanieplageman(at)gmail(dot)com> wrote:
>
> Thanks for the patch...
>
> > Attached is a patch set which refactors BitmapHeapScan such that it
> > can use the streaming read API [1]. It also resolves the long-standing
> > FIXME in the BitmapHeapScan code suggesting that the skip fetch
> > optimization should be pushed into the table AMs. Additionally, it
> > moves table scan initialization to after the index scan and bitmap
> > initialization.
> >
> > patches 0001-0002 are assorted cleanup needed later in the set.
> > patches 0003 moves the table scan initialization to after bitmap creation
> > patch 0004 is, I think, a bug fix. see [2].
> > patches 0005-0006 push the skip fetch optimization into the table AMs
> > patches 0007-0009 change the control flow of BitmapHeapNext() to match
> > that required by the streaming read API
> > patch 0010 is the streaming read code not yet in master
> > patch 0011 is the actual bitmapheapscan streaming read user.
> >
> > patches 0001-0009 apply on top of master but 0010 and 0011 must be
> > applied on top of a commit before a 21d9c3ee4ef74e2 (until a rebased
> > version of the streaming read API is on the mailing list).
>
> I followed your lead and applied them to 6a8ffe812d194ba6f4f26791b6388a4837d17d6c. `make check` worked fine, though I expect you know that already.

Thanks for taking a look!

> > The caveat is that these patches introduce breaking changes to two
> > table AM functions for bitmapheapscan: table_scan_bitmap_next_block()
> > and table_scan_bitmap_next_tuple().
>
> You might want an independent perspective on how much of a hassle those breaking changes are, so I took a stab at that. Having written a custom proprietary TAM for postgresql 15 here at EDB, and having ported it and released it for postgresql 16, I thought I'd try porting it to the the above commit with your patches. Even without your patches, I already see breaking changes coming from commit f691f5b80a85c66d715b4340ffabb503eb19393e, which creates a similar amount of breakage for me as does your patches. Dealing with the combined breakage might amount to a day of work, including testing, half of which I think I've already finished. In other words, it doesn't seem like a big deal.
>
> Were postgresql 17 shaping up to be compatible with TAMs written for 16, your patch would change that qualitatively, but since things are already incompatible, I think you're in the clear.

Oh, good to know! I'm very happy to have the perspective of a table AM
author. Just curious, did your table AM implement
table_scan_bitmap_next_block() and table_scan_bitmap_next_tuple(),
and, if so, did you use the TBMIterateResult? Since it is not used in
BitmapHeapNext() in my version, table AMs would have to change how
they use TBMIterateResults anyway. But I assume they could add it to a
table AM specific scan descriptor if they want access to a
TBMIterateResult of their own making in both
table_san_bitmap_next_block() and next_tuple()?

- Melanie