Subject: Re: BitmapHeapScan streaming read user and prelim refactoring

From: Melanie Plageman <[email-masked]>
Date: Sun, 24 Mar 2024 16:12:17 -0400

On Sun, Mar 24, 2024 at 2:22 PM Tomas Vondra
<[email-masked]> wrote:
>
> On 3/24/24 18:38, Melanie Plageman wrote:
> > I haven't had a chance yet to reproduce the regressions you saw in the
> > streaming read user patch or to look closely at the performance results.
>
> So you tried to reproduce it and didn't hit the issue? Or didn't have
> time to look into that yet? FWIW with v7 it failed almost immediately
> (only a couple queries until hitting one triggering the issue), but v9
> that's not the case (hundreds of queries without an error).

I haven't started trying to reproduce it yet.

> I however wonder what the plan with these patches is - do we still plan
> to get some of this into v17? It seems to me we're getting uncomfortably
> close to the end of the cycle, with a fairly incomplete idea of how it
> affects performance.
>
> Which is why I've been focusing more on the refactoring patches (up to
> 0015), to make sure those don't cause regressions if committed. And I
> think that's generally true.

Thank you for testing the refactoring patches with this in mind! Out
of the refactoring patches, I think there is a subset of them that
have independent value without the streaming read user. I think it is
worth committing the first few patches because they remove a table AM
layering violation. IMHO, all of the patches up to "Make
table_scan_bitmap_next_block() async friendly" make the code nicer and
better. And, if folks like the patch "Remove
table_scan_bitmap_next_block()", then I think I could rebase that back
in on top of "Make table_scan_bitmap_next_block() async friendly".
This would mean table AMs would only have to implement one callback
(table_scan_bitmap_next_tuple()) which I also think is a net
improvement and simplification.

The other refactoring patches may not be interesting without the
streaming read user.

> But for the main StreamingRead API the situation is very different.

My intent for the bitmapheapscan streaming read user was to get it
into 17, but I'm not sure that looks likely. The main issues Thomas is
looking into right now are related to regressions for a fully cached
scan (noticeable with the pg_prewarm streaming read user). With all of
these fixed, I anticipate we will still see enough behavioral
differences with the bitmapheap scan streaming read user that it may
not be committable in time. Though, I have yet to work on reproducing
the regressions with the BHS streaming read user mostly because I was
focused on getting the refactoring ready and not as much because the
streaming read API is unstable.

- Melanie

---

From: Tomas Vondra <[email-masked]>
Date: Sun, 24 Mar 2024 22:59:25 +0100

> The other refactoring patches may not be interesting without the

I admit not reviewing the individual patches very closely yet, but this
matches how I understood them - that at least some are likely an
improvement on their own, not just as a refactoring preparing for the
switch to streaming reads.

We only have ~2 weeks left, so it's probably time to focus on getting at
least those improvements committed. I see Heikki was paying way more
attention to the patches than me, though ...

BTW when you say "up to 'Make table_scan_bitmap_next_block() async
friendly'" do you mean including that patch, or that this is the first
patch that is not one of the independently useful patches.

(I took a quick look at the first couple patches and I appreciate that
you keep separate patches with small cosmetic changes to keep the actual
patch smaller and easier to understand.)

> streaming read API is unstable.

I don't have a very good intuition regarding impact of the streaming API
patch on performance. I haven't been following that thread very closely,
but AFAICS there wasn't much discussion about that - perhaps it happened
offlist, not sure. So who knows, really?

Which is why I started looking at this patch instead - it seemed easier
to benchmark with a somewhat realistic workload.

But yeah, there certainly were significant behavior changes, and it's
unlikely that whatever Thomas did in v8 made them go away.

FWIW I certainly am *not* suggesting there must be no behavior changes,
that's simply not possible. I'm not even suggesting no queries must get
slower - given the dependence on storage, I think some regressions are
pretty much inevitable. But it's still be good to know the regressions
are reasonably rare exceptions rather than the common case, and that's
not what I'm seeing ...

regards

-- 
Tomas Vondra
EnterpriseDB: http://www.enterprisedb.com
The Enterprise PostgreSQL Company

---

From: Melanie Plageman <[email-masked]>
Date: Sun, 24 Mar 2024 18:37:20 -0400

> patch that is not one of the independently useful patches.

I think the code is easier to understand with "Make
table_scan_bitmap_next_block() async friendly". Prior to that commit,
table_scan_bitmap_next_block() could return false even when the bitmap
has more blocks and expects the caller to handle this and invoke it
again. I think that interface is very confusing. The downside of the
code in that state is that the code for prefetching is still in the
BitmapHeapNext() code and the code for getting the current block is in
the heap AM-specific code. I took a stab at fixing this in v9's 0013,
but the outcome wasn't very attractive.

What I will do tomorrow is reorder and group the commits such that all
of the commits that are useful independent of streaming read are first
(I think 0014 and 0015 are independently valuable but they are on top
of some things that are only useful to streaming read because they are
more recently requested changes). I think I can actually do a bit of
simplification in terms of how many commits there are and what is in
each. Just to be clear, v9 is still reviewable. I am just going to go
back and change what is included in each commit.

> patch smaller and easier to understand.)

Thanks!