Horton

Re: index prefetching

Peter Geoghegan <pg@bowt.ie>

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From: Peter Geoghegan <pg@bowt.ie>
To: Tomas Vondra <tomas@vondra.me>
Cc: Andres Freund <andres@anarazel.de>, Robert Haas <robertmhaas@gmail.com>, Melanie Plageman <melanieplageman@gmail.com>, PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>, Georgios <gkokolatos@protonmail.com>, Thomas Munro <thomas.munro@gmail.com>, Konstantin Knizhnik <knizhnik@garret.ru>, Dilip Kumar <dilipbalaut@gmail.com>
Date: 2025-07-16T14:45:14Z
Lists: pgsql-hackers

Commits

Same data as JSON: GET /api/v1/messages/:b64id/commits the thread's linked commits as JSON, with link sources. API reference →

  1. aio: io_uring: Trigger async processing for large IOs

  2. read stream: Split decision about look ahead for AIO and combining

  3. read_stream: Only increase read-ahead distance when waiting for IO

  4. read_stream: Prevent distance from decaying too quickly

  5. Reduce ExecSeqScan* code size using pg_assume()

  6. Fix rare bug in read_stream.c's split IO handling.

  7. Fix multiranges to behave more like dependent types.

  8. Add EXPLAIN (MEMORY) to report planner memory consumption

  9. Optimize nbtree backward scan boundary cases.

  10. Increment xactCompletionCount during subtransaction abort.

  11. Add nbtree Valgrind buffer lock checks.

  12. Add nbtree high key "continuescan" optimization.

  13. Reduce pinning and buffer content locking for btree scans.

  14. Teach btree to handle ScalarArrayOpExpr quals natively.

On Wed, Jul 16, 2025 at 10:37 AM Tomas Vondra <tomas@vondra.me> wrote:
> What sounds weird? That the read_stream works like a stream of blocks,
> or that it can't do "pause" and we use "reset" as a workaround?

The fact that prefetch distance is in any way affected by a temporary
inability to return more blocks. Just starting from scratch seems
particularly bad.

Doesn't that mean that it's simply impossible for us to remember
ramping up the distance on an earlier leaf page? There is nothing
about leaf page boundaries that should be meaningful to the read
stream/our heap accesses.

I get that index characteristics could be the limiting factor,
especially in a world where we're not yet eagerly reading leaf pages.
But that in no way justifies just forgetting about prefetch distance
like this.

> >> In an ideal world we'd have a function that'd "pause" the stream,
> >> without resetting the distance etc. But we don't have that, and the
> >> reset thing was suggested to me as a workaround.
> >
> > Does the "complex" patch require a similar workaround? Why or why not?
> >
>
> I think it'll need to do something like that in some cases, when we need
> to limit the number of leaf pages kept in memory to something sane.

That's the only reason? The memory usage for batches?

That doesn't seem like a big deal. It's something to keep an eye on,
but I see no reason why it'd be particularly difficult.

Doesn't this argue for the "complex" patch's approach?

-- 
Peter Geoghegan