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: 2025-08-14T19:15:02Z

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 14, 2025 at 2:53 PM Andres Freund <andres@anarazel.de> wrote:
> I think this is just an indicator of being IO bound.

Then why does the exact same pair of runs show "I/O Timings: shared
read=194.629" for the sequential table backwards scan (with total
execution time 1132.360 ms), versus "I/O Timings: shared read=352.88"
(with total execution time 697.681 ms) for the random table backwards
scan?

Obviously it is hard to believe that the query with shared
read=194.629 is one that is naturally much more I/O bound than another
similar query that shows shared read=352.88. What "I/O Timings" shows
more or less makes sense to me already -- it just doesn't begin to
explain why *overall query execution* is much slower when scanning
backwards sequentially.

>   I'd see what changes if you temporarily reduce
>   /sys/block/nvme6n1/queue/max_sectors_kb to a smaller size.

I reduced max_sectors_kb from 128 to 8. That had no significant effect.

> Could you show iostat for both cases?

iostat has lots of options. Can you be more specific?

-- 
Peter Geoghegan