Re: Streaming read-ready sequential scan code

On Wed, Feb 28, 2024 at 12:30 PM Melanie Plageman
<melanieplageman(at)gmail(dot)com> wrote:
>
> On Mon, Feb 26, 2024 at 03:56:57PM -0500, Melanie Plageman wrote:
> > On Mon, Feb 19, 2024 at 6:05 PM Melanie Plageman
> > <melanieplageman(at)gmail(dot)com> wrote:
> > >
> > > On Mon, Jan 29, 2024 at 4:17 PM Melanie Plageman
> > > <melanieplageman(at)gmail(dot)com> wrote:
> > > >
> > > > There is an outstanding question about where to allocate the
> > > > PgStreamingRead object for sequential scans
> > >
> > > I've written three alternative implementations of the actual streaming
> > > read user for sequential scan which handle the question of where to
> > > allocate the streaming read object and how to handle changing scan
> > > direction in different ways.
> > >
> > > Option A) https://github.com/melanieplageman/postgres/tree/seqscan_pgsr_initscan_allocation
> > > - Allocates the streaming read object in initscan(). Since we do not
> > > know the scan direction at this time, if the scan ends up not being a
> > > forwards scan, the streaming read object must later be freed -- so
> > > this will sometimes allocate a streaming read object it never uses.
> > > - Only supports ForwardScanDirection and once the scan direction
> > > changes, streaming is never supported again -- even if we return to
> > > ForwardScanDirection
> > > - Must maintain a "fallback" codepath that does not use the streaming read API
> >
> > Attached is a version of this patch which implements a "reset"
> > function for the streaming read API which should be cheaper than the
> > full pg_streaming_read_free() on rescan. This can easily be ported to
> > work on any of my proposed implementations (A/B/C). I implemented it
> > on A as an example.
>
> Attached is the latest version of this patchset -- rebased in light of
> Thomas' updatees to the streaming read API [1]. I have chosen the
> approach I think we should go with. It is a hybrid of my previously
> proposed approaches.

While investigating some performance concerns, Andres pointed out that
the members I add to HeapScanDescData in this patch push rs_cindex and
rs_ntuples to another cacheline and introduce a 4-byte hole. Attached
v4's HeapScanDescData is as well-packed as on master and its members
are reordered so that rs_cindex and rs_ntuples are back on the second
cacheline of the struct's data.

- Melanie