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: 2025-08-14T19:45:26Z

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 3:15 PM Peter Geoghegan <pg@bowt.ie> wrote:
> 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?

If you're interested in trying this out for yourself, I've pushed my
working branch here:

https://github.com/petergeoghegan/postgres/tree/index-prefetch-batch-v1.2

Note that the test case you'll run is added by the most recent commit:

https://github.com/petergeoghegan/postgres/commit/c9ceb765f3b138f53b7f1fdf494ba7c816082aa1

Run microbenchmarks/random_backwards_weird.sql to do an initial load
of both of the tables. Then run
microbenchmarks/queries_random_backwards_weird.sql to actually run the
relevant queries. There are 4 such queries, but only the 2 backwards
scan queries really seem relevant.

-- 
Peter Geoghegan