Re: index prefetching

Peter Geoghegan <pg@bowt.ie>

From: Peter Geoghegan <pg@bowt.ie>

To: Andres Freund <andres@anarazel.de>

Cc: Tomas Vondra <tomas@vondra.me>, Thomas Munro <thomas.munro@gmail.com>, Nazir Bilal Yavuz <byavuz81@gmail.com>, Robert Haas <robertmhaas@gmail.com>, Melanie Plageman <melanieplageman@gmail.com>, PostgreSQL Hackers <pgsql-hackers@lists.postgresql.org>, Georgios <gkokolatos@protonmail.com>, Konstantin Knizhnik <knizhnik@garret.ru>, Dilip Kumar <dilipbalaut@gmail.com>

Date: 2025-08-29T01:52:48Z

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 →

aio: io_uring: Trigger async processing for large IOs
- a9ee66881744 19 (unreleased) landed
read stream: Split decision about look ahead for AIO and combining
- 8ca147d582a5 19 (unreleased) landed
read_stream: Only increase read-ahead distance when waiting for IO
- f63ca3379025 19 (unreleased) landed
read_stream: Prevent distance from decaying too quickly
- 6e36930f9aaf 19 (unreleased) landed
Reduce ExecSeqScan* code size using pg_assume()
- b227b0bb4e03 19 (unreleased) cited
Fix rare bug in read_stream.c's split IO handling.
- b421223172a2 19 (unreleased) cited
Fix multiranges to behave more like dependent types.
- 3e8235ba4f9c 17.0 cited
Add EXPLAIN (MEMORY) to report planner memory consumption
- 5de890e3610d 17.0 cited
Optimize nbtree backward scan boundary cases.
- c9c0589fda0e 17.0 cited
Increment xactCompletionCount during subtransaction abort.
- 90c885cdab8b 14.0 cited
Add nbtree Valgrind buffer lock checks.
- 4a70f829d86c 14.0 cited
Add nbtree high key "continuescan" optimization.
- 29b64d1de7c7 12.0 cited
Reduce pinning and buffer content locking for btree scans.
- 2ed5b87f96d4 9.5.0 cited
Teach btree to handle ScalarArrayOpExpr quals natively.
- 9e8da0f75731 9.2.0 cited

On Thu, Aug 28, 2025 at 9:10 PM Andres Freund <andres@anarazel.de> wrote:
> Same.  Tomas, could you share what you applied?

Tomas posted a self-contained patch to the list about an hour ago?

> > I'm not sure that Thomas'/your patch to ameliorate the problem on the
> > read stream side is essential here. Perhaps Andres can just take a
> > look at the test case + feature branch, without the extra patches.
> > That way he'll be able to see whatever the immediate problem is, which
> > might be all we need.
>
> It seems caused to a significant degree by waiting at low queue depths.  If I
> comment out the stream->distance-- in read_stream_start_pending_read() the
> regression is reduced greatly.

IIUC, that is very roughly equivalent to what the patch actually does.

The fastest configuration of all, independent of io_method, is
"enable_indexscan_prefetch=off". So it's hard to believe that the true
underlying problem is low queue depth. Though I certainly don't doubt
that higher queue depths will help *when io_method=worker*.

-- 
Peter Geoghegan