Re: EXPLAIN: showing ReadStream / prefetch stats

On 9/22/26 21:02, Nikolay Samokhvalov wrote:
> On Tue, Sep 22, 2026 at 8:23 AM Tomas Vondra <tomas(at)vondra(dot)me> wrote:
>> It's not clear to me why this would be desirable. The query execution
>> clearly *does* perform the I/O, even if the buffers end up not being
>> used. Hiding that would be very confusing / misleading.
>>
>> In other words, I/O and buffers may disagree - that's not a bug. Those
>> counters are tracking different things.
>
> Thanks all. Agreed that issued I/O should remain accounted for.
>
> Is Prefetch avg/max also intended to include buffers discarded by
> read_stream_reset()? If so, agreed, the patch is wrong. The comment above
> read_stream_count_prefetch() says it is called once per buffer returned
> to the consumer.
>

Yeah, that's a good point. The comment is misleading. I didn't consider
that read_stream_next_buffer() is called from _reset() when I wrote it.

Will think how to clarify it.

regards

--
Tomas Vondra