Horton

Re: index prefetching

Peter Geoghegan <pg@bowt.ie>

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From: Peter Geoghegan <pg@bowt.ie>
To: Andres Freund <andres@anarazel.de>
Cc: Tomas Vondra <tomas@vondra.me>, Alexandre Felipe <o.alexandre.felipe@gmail.com>, 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: 2026-02-18T01:11:24Z
Lists: pgsql-hackers 
On Tue, Feb 17, 2026 at 7:31 PM Andres Freund <andres@anarazel.de> wrote:
> Interestingly, I can't reproduce a regression compared to index prefetching
> being disabled.
>
> If I evict just prefetch_customers between runs, I see prefetch + no-yield
> being the fastest by a good amount.
>
> If I evict prefetch_customers as well as prefetch_customers_pkey, yielding
> wins, but only just about. Which I guess makes sense, the index reads are
> synchronous random reads, and we do more of those if we prefetch too
> aggressively.

That can't have been a factor when I ran the query (which is pretty
obvious from the EXPLAIN ANALYZE output).

> I ran the queries both with pgbench (in a script that evicts the buffers, but
> then just looks at the per-statement time for the SELECT, 30 iterations) and
> separately interactively with EXPLAIN ANALYZE to get IO stats.
>
>
> This is with debug_io_direct=data, were you measuring this without DIO? If so,
> was the data in the page cache or did you evict it from there?

I rarely use debug_io_direct=data for any of my testing. The standard
for me is to use buffered IO + io_uring, while prewarming all indexes
and evicting all heap relations. And by evicting the OS filesystem
cache before each run.

FWIW, the version of the patch you're using is slightly different to
the one I have here. Since I worked on unrelated issues with things
like the cost of rescans with nestloop joins. Another difference is
that the memory allocation for the VM cache is now combined with the
main batch alloc, which seems to be more cache efficient. And saves us
memory for plain index scans.

> We really should add a function to pg_prewarm (or pg_buffercache, or ...) that
> evicts pages in a targeted way from the kernel page cache... Flushing the
> entire kernel pagecache leads to undesirable noise, because it also evicts
> filesystem metadata (on somefilesystems at least) etc.

Yeah, having to flush the kernel page cache has been really inconvenient.

> FWIW, I got a crash in a mark-restore query. I think I see the problem:
>
>         /*
>          * Release all currently loaded batches, being sure to avoid freeing
>          * markBatch (unless called with complete, where we're supposed to)
>          */
>         for (uint8 i = batchringbuf->headBatch; i != batchringbuf->nextBatch; i++)
>         {
>                 IndexScanBatch batch = index_scan_batch(scan, i);
>
>                 if (complete || batch != markBatch)
>                 {
>                         markBatchFreed = (batch == markBatch);
>                         tableam_util_free_batch(scan, batch);
>                 }
>         }
>
>         if (complete && markBatch != NULL && !markBatchFreed)
>         {
>                 /*
>                  * We didn't free markBatch because it was no longer loaded in ring
>                  * buffer.  Do so now instead.
>                  */
>                 tableam_util_free_batch(scan, markBatch);
>         }
>
> If, in the loop, there's a batch after the markBatch in the ring, it'll reset
> markBatchFreed to false. Which then leads to the batch being freed a second
> time.

Oops.


-- 
Peter Geoghegan

Commits

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

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

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

  4. heapam: Keep buffer pins across index scan resets.

  5. heapam: Track heap block in IndexFetchHeapData.

  6. Move heapam_handler.c index scan code to new file.

  7. Rename heapam_index_fetch_tuple argument for clarity.

  8. Optimize fast-path FK checks with batched index probes

  9. read_stream: Prevent distance from decaying too quickly

  10. read_stream: Issue IO synchronously while in fast path

  11. bufmgr: Return whether WaitReadBuffers() needed to wait

  12. aio: io_uring: Allow IO methods to check if IO completed in the background

  13. bufmgr: Make UnlockReleaseBuffer() more efficient

  14. Add fake LSN support to hash index AM.

  15. Make IndexScanInstrumentation a pointer in executor scan nodes.

  16. Use fake LSNs to improve nbtree dropPin behavior.

  17. Move fake LSN infrastructure out of GiST.

  18. Use simplehash for backend-private buffer pin refcounts.

  19. nbtree: Avoid allocating _bt_search stack.

  20. bufmgr: Fix use of wrong variable in GetPrivateRefCountEntrySlow()

  21. Conditional locking in pgaio_worker_submit_internal

  22. Reduce ExecSeqScan* code size using pg_assume()

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

  24. Remove HeapBitmapScan's skip_fetch optimization

  25. Optimize nbtree backwards scans.

  26. Fix multiranges to behave more like dependent types.

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

  28. Optimize nbtree backward scan boundary cases.

  29. Increment xactCompletionCount during subtransaction abort.

  30. Add nbtree Valgrind buffer lock checks.

  31. Add nbtree high key "continuescan" optimization.

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

  33. Teach btree to handle ScalarArrayOpExpr quals natively.