Re: explain analyze rows=%.0f

On 11.02.2025 20:41, Robert Haas wrote:
> On Tue, Feb 11, 2025 at 12:14 PM Andrei Lepikhov <lepihov(at)gmail(dot)com> wrote:
>> I support the idea in general, but I believe it should be expanded to
>> cover all cases of parameterised plan nodes. Each rescan iteration may
>> produce a different number of tuples, and rounding can obscure important
>> data.
>>
>> For example, consider five loops of a scan node: the first loop returns
>> nine tuples, and each other - zero tuples. When we calculate the
>> average, 9 divided by 5 equals 1.8. This results in an explanation that
>> indicates "rows = 1," masking almost 40% of the data.
>>
>> Now, if we apply the same two loops but have a total of 900,000 tuples,
>> then 400,000 masked tuples represent a significant portion of the data.
>>
>> Moreover, switching to a floating-point type for row explanations in
>> each parameterised node would provide a more comprehensive view and add
>> valuable information about the parameterisation of the node, which may
>> not be immediately apparent.
> I agree strongly with all of this. I believe we should just implement
> what was agreed here:
>
> https://www.postgresql.org/message-id/21013.1243618236%40sss.pgh.pa.us
>
> Let's just display 2 fractional digits when nloops>1, else 0, and call it good.
>

Thank you for your review!

With such example, it's hard to disagree with it. This would really add
valuable information. Taking all opinions into account, I have updated
the patch v8. I have also included a check for the case where there are
only zeros after the decimal point. We do not want to clutter the rows
with unnecessary zeros.

--
Best regards,
Ilia Evdokimov,
Tantor Labs LLC.