Horton

Re: Streaming read-ready sequential scan code

Melanie Plageman <melanieplageman@gmail.com>

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From: Melanie Plageman <melanieplageman@gmail.com>
To: Pg Hackers <pgsql-hackers@postgresql.org>
Cc: David Rowley <dgrowleyml@gmail.com>, Andres Freund <andres@anarazel.de>, Thomas Munro <thomas.munro@gmail.com>, Heikki Linnakangas <hlinnaka@iki.fi>
Date: 2024-02-26T20:56:57Z
Lists: pgsql-hackers

Attachments

On Mon, Feb 19, 2024 at 6:05 PM Melanie Plageman
<melanieplageman@gmail.com> wrote:
>
> On Mon, Jan 29, 2024 at 4:17 PM Melanie Plageman
> <melanieplageman@gmail.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.

- Melanie

Commits

  1. Fix unfairness in all-cached parallel seq scan.

  2. Fix if/while thinko in read_stream.c edge case.

  3. Increase default vacuum_buffer_usage_limit to 2MB.

  4. Allow BufferAccessStrategy to limit pin count.

  5. Improve read_stream.c's fast path.

  6. Secondary refactor of heap scanning functions

  7. Preliminary refactor of heap scanning functions

  8. Add VACUUM/ANALYZE BUFFER_USAGE_LIMIT option