Re: Show 'Rows Removed by Filter' with two fractional digits in EXPLAIN

On 06/11/2026 14:17, Ilia Evdokimov wrote:
> On 11/6/26 11:57, Andrei Lepikhov wrote:
>> The patch is a single-line change in show_instrumentation_count() plus two
>> tests. It is a concept, not a finished patch: I have not regenerated the other
>> expected outputs, where many values will change from "N" to "N.00" - you can
>> find a sketch for that in 0002-* file.
>
> If we switch to fractional row counts, we should also remember to update the
> relevant examples in the doc, including "Rows Removed by Filter", "Rows Removed
> by Index Recheck", "Rows Removed by Conflict Filter", "Rows Removed by Join
> Filter".
>
> Regardless of whether we change this output format, it would be be useful to
> document in the "Using EXPLAIN" section (perform.sgml), in the paragraph
> describing "Rows Removed", that these counts are averages per execution of the
> node. This is already the case, but the paragraph does not mention it.
Ok, my AI coding agent detects such places quite nicely. See the patch v0-0002-*
in attachment.

>> rule as actual rows. The options I see are:
>>
>> 1. Always print two digits, as actual rows do now.
>> 2. Leave the math to users and print exact totals without dividing by nloops.
>>
>> The second approach is a much bigger change, with obvious pros and cons.
>> The first one is simpler and more conservative, and it is what the attached
>> patch does. In [1], depesz describes how the switch to fractional actual rows
>> affected his explain parser, and it needed only a small fix. So I expect this
>> change to be similarly harmless for tools.
>
> I also prefer the first option, as it is more conservative and keeps these
> counters consistent with actual rows.
Hmm, I wouldn't say that I prefer the first approach. Reading explains quite
frequently that I'd prefer sums on all cycles for sure - sometimes huge work is
hidden in the third or even sixth digit after the point - because it is the
first non-zero digit. But I understand the reasoning and agree - it is a
contradictory question. So, No.1 is just a compromise.

--
regards, Andrei Lepikhov,
pgEdge