Horton

Re: Streaming read-ready sequential scan code

Andres Freund <andres@anarazel.de>

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From: Andres Freund <andres@anarazel.de>
To: Thomas Munro <thomas.munro@gmail.com>
Cc: Melanie Plageman <melanieplageman@gmail.com>, David Rowley <dgrowleyml@gmail.com>, Heikki Linnakangas <hlinnaka@iki.fi>, Pg Hackers <pgsql-hackers@postgresql.org>
Date: 2024-04-07T22:12:29Z
Lists: pgsql-hackers 
Hi,

On 2024-04-08 09:36:59 +1200, Thomas Munro wrote:
> I've been on the fence about that flag for sequential scan...  Some
> days I want to consider changing to READ_STREAM_DEFAULT and relying on
> our "anti-heuristics" to suppress advice, which would work out the
> same in most cases but might occasionally win big.

Agreed, it's pretty easy to end up with a fairly "fragmented" set of a
relation's buffers in s_b.  OTOH, there might not be any need for the
heuristic if we actually trigger reads asynchronously.


> BTW looking at the branching in read-stream user patches that have an
> initialisation step like yours, I wonder if it might every make sense
> to be able to change the callback on the fly from inside the callback,
> so that you finish up with a branchless one doing most of the work.  I
> have no idea if it's worth it...

I was wondering about that too, I dislike those branches. But instead of
changing the callback, it seems like a better design would be to have another
dedicated callback for that?  There already is a dedicated branch for the
"just starting up" path in read_stream_next_buffer(), so it'd be pretty much
free to call another callback there.

Greetings,

Andres Freund

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