Re: index prefetching

On Fri, Feb 27, 2026 at 4:18 AM Andres Freund <andres(at)anarazel(dot)de> wrote:

> Hi,
>
> I'm planning to do some reviewing in the next days. In preparation I just
> retried a benchmark and saw some odd results. After a while I was able to
> reproduce even with a simpler setup:
>

> I'm planning to do some reviewing in the next days. In preparation I just

> retried a benchmark and saw some odd results.

Since we are talking about results I will share mine too :)

The bottomline is: Prefetch is working, but it might make some things
slower.

It is obvious that this should better exploit IO for one single heavy
query, in

one single table.

It is not so obvious, to me, how this would behave when there are multiple

concurrent queries. It is not so obvious how this will impact when multiple

tables are queried at the same time. My feeling is that it should greatly

improve on a disk with a mechanical head, if it performs the same reads

reducing the number of times it has to jump from one to another. Is there
much

interest in special optimisations for those or is the focus more on SSDs?

On my previous review I wasted way to much time trying to improve
read_stream,

to end up getting just some mixed results. This time I tried to step back
and

try to look at various functions that could have changed. Initially I tried

compiler function instrumentation, but then the profiling overhead of 33k

functions dominated.

This time what I did (1) added a indexscan_prefetch_distance, maybe a better

name would be just prefetch_distance, it limits the growth of distance in

read_stream (distance-limit.diff). (2) captured execution statistics for 15

functions (profiling-instrumentation.diff). At exit each process will
create a

log with its configuration and call statistics.

The benchmark was with full index scan on a sequential column, executed

repeatedly and no cache eviction: buffer hit path.

BENCHMARK RESULTS

MacOS in normal (for me) use

Prefetch Avg Time Min Time Max Time

------------------------------------------------

off 6.03s 5.12s 11.70s

1 59.44s 25.33s 257.60s

4 19.74s 12.66s 44.36s

16 11.87s 7.49s 19.13s

64 8.77s 6.05s 13.97s

128 6.40s 4.33s 11.74s

MacOS idle, after reboot

Prefetch Avg Time Min Time Max Time

------------------------------------------------

off 2.17s 2.12s 2.26s

1 5.53s 5.44s 5.57s

4 3.17s 3.04s 3.39s

16 3.13s 3.04s 3.29s

64 2.82s 2.66s 2.88s

128 2.83s 2.69s 2.90s

Docker on MacOS, idle, after reboot

Prefetch Avg Time Min Time Max Time

------------------------------------------------

off 1.38s 1.36s 1.46s

1 3.65s 3.56s 3.70s

4 2.00s 1.98s 2.09s

16 1.56s 1.53s 1.59s

64 1.29s 1.25s 1.33s

128 1.28s 1.26s 1.32s

Docker on Linux

Prefetch Avg Time Min Time Max Time

------------------------------------------------

off 6.07s 5.92s 6.29s

1 6.85s 6.67s 7.04s

4 6.26s 6.10s 6.41s

16 6.14s 5.95s 6.30s

64 5.74s 5.62s 5.91s

128 5.72s 5.63s 5.86s

The linux execution presented very little degradation. On MacOS host the

degradation was more noticeable than on MacOS docker running a debian,

suggesting that software ecosystem contributes, docker on MacOS (arm), was

slower than docker on a native linux (x86_64), here I could be it is CPU

architecture or OS kernel differences.

WHAT CHANGED

The benchmark will produced, 195 autovac_worker, and 3293 backend and one

bgworker log. For prefetch off the number of calls is constant. For
prefetch on

they vary widely, but I am looking at the total time per function, assuming

that the differences in the number of calls changes only how the work was

partitioned but the final work was the same.

With Docker version 28.3.0, build 38b7060, Python 3.10.18

$ docker compose up --build benchmark

$ docker cp docker-postgres-1:/tmp/profiling ./docker-profiling

$ python compare_profiles.py docker-profiling

Function off,d=0 on,d=128 Diff % z-statistic

------------------------------------------------------------------------

read_stream_next_buffer 0.0 3944.9 +3944.9 NEW% +654.88

read_stream_look_ahead 0.0 2999.3 +2999.3 NEW% +624.00

WaitReadBuffers 98.3 754.6 +656.3 +667.7% +414.56

_bt_next 748.7 1072.8 +324.1 +43.3% +20.35

heapam_batch_getnext 788.4 1114.6 +326.2 +41.4% +20.18

btgetbatch 777.0 1096.7 +319.7 +41.2% +20.14

IndexNext 17031.7 10400.3 -6631.5 -38.9% -249.51

_bt_first 17.2 13.0 -4.2 -24.6% 10.56

Function off,d=0 on,d=1 Diff % z-statistic

------------------------------------------------------------------------

read_stream_look_ahead 0.0 28135.9 +28135.9 NEW N/A

read_stream_next_buffer 0.0 199245.7 +199245.7 NEW N/A

IndexNext 17031.7 211861.0 +194829.2 12.4x +283.00

WaitReadBuffers 98.3 169641.9 +169543.6 1724x +275.63

heapam_index_fetch_tuple 13564.5 205828.2 +192263.7 15x +172.56

heapam_batch_getnext 788.4 1944.0 +1155.6 +146.6% +25.39

_bt_next 748.7 1833.9 +1085.2 +144.9% +24.85

btgetbatch 777.0 1881.2 +1104.2 +142.1% +24.74
_bt_first 17.2 19.3 +2.1 +12.0% +10.33

PS.: The docker environment cache eviction requires adjustments.