Re: BitmapHeapScan streaming read user and prelim refactoring

On 4/7/24 15:11, Melanie Plageman wrote:
> On Sun, Apr 7, 2024 at 7:38 AM Tomas Vondra
> <tomas(dot)vondra(at)enterprisedb(dot)com> wrote:
>>
>> On 4/7/24 06:17, Melanie Plageman wrote:
>>>> What bothers me on 0006-0008 is that the justification in the commit
>>>> messages is "future commit will do something". I think it's fine to have
>>>> a separate "prepareatory" patches (I really like how you structured the
>>>> patches this way), but it'd be good to have them right before that
>>>> "future" commit - I'd like not to have one in v17 and then the "future
>>>> commit" in v18, because that's annoying complication for backpatching,
>>>> (and probably also when implementing the AM?) etc.
>>>
>>> Yes, I was thinking about this also.
>>>
>>
>> Good we're on the same page.
>
> Having thought about this some more I think we need to stop here for
> 17. v20-0001 and v20-0002 both make changes to the table AM API that
> seem bizarre and unjustifiable without the other changes. Like, here
> we changed all your parameters because someday we are going to do
> something! You're welcome!
>

OK, I think that's essentially the "temporary breakage" that should not
span multiple releases, I mentioned ~yesterday. I appreciate you're
careful about this.

> Also, the iterators in the TableScanDescData might be something I
> could live with in the source code for a couple months before we make
> the rest of the changes in July+. But, adding them does push the
> TableScanDescData->rs_parallel member into the second cacheline, which
> will be true in versions of Postgres people are using for years. I
> didn't perf test, but seems bad.
>

I haven't though about how it affects cachelines, TBH. I'd expect it to
have minimal impact, because while it makes this struct larger it should
make some other struct (used in essentially the same places) smaller. So
I'd guess this to be a zero sum game, but perhaps I'm wrong.

For me the main question was "Is this the right place for this, even if
it's only temporary?"

> So, yes, unfortunately, I think we should pick up on the BHS saga in a
> few months. Or, actually, we should start focusing on that parallel
> BHS + 0 readahead bug and whether or not we are going to fix it.
>

Yes, the July CF is a good time to focus on this, early in the cycle.

> Sorry for the about-face.
>

No problem. I very much prefer this over something that may not be quite
ready yet.

regards

--
Tomas Vondra
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