Message-ID: <a51fc17e-8702-4928-863b-ea3eecf940bf@tantorlabs.com>
Date: Wed, 12 Feb 2025 13:10:34 +0300
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Subject: Re: explain analyze rows=%.0f
To: Andrei Lepikhov <lepihov@gmail.com>, Tom Lane <tgl@sss.pgh.pa.us>,
 Robert Haas <robertmhaas@gmail.com>
Cc: Matheus Alcantara <matheusssilv97@gmail.com>,
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 Amit Kapila <amit.kapila16@gmail.com>, Justin Pryzby <pryzby@telsasoft.com>,
 Peter Geoghegan <pg@bowt.ie>, vignesh C <vignesh21@gmail.com>,
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From: Ilia Evdokimov <ilya.evdokimov@tantorlabs.com>
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On 12.02.2025 11:54, Andrei Lepikhov wrote:
> May we agree on a more general formula to print at least two 
> meaningful digits if we have a fractional part?
>
> Examples:
> - actual rows = 2, nloops = 2 -> rows = 1
> - actual rows = 9, nloops = 5 -> rows = 1.8
> - actual rows = 101, nloops = 100 -> rows = 1.0
> - actual rows = 99, nloops = 1000000 -> rows = 0.000099
>
> It may guarantee that an EXPLAIN exposes most of the data passed the 
> node, enough to tangle it with actual timing and tuples at the upper 
> levels of the query.

I think the idea of keeping two significant digits after the decimal 
point is quite reasonable. The thing is, rows=0.000001 or something 
similar can only occur when loops is quite large. If we show the order 
of magnitude in rows, it will be easier for the user to estimate the 
order of total rows. For example, if we see this:

rows=0.000056 loops=4718040

the user can quickler approximate the order of total rows for analyzing 
the upper levels of the query.

However, keep in mind that I am against using the E notation, as many 
users have mentioned that they are not mathematicians and are not 
familiar with the concept of "E".


>
>>
>> I suggest that when thinking about what to change here,
>> you start by considering how you'd adjust the docs at
>> https://www.postgresql.org/docs/devel/using-explain.html
>> to explain the new behavior.  If you can't explain it
>> clearly for users, then maybe it's not such a great idea.
> Agree
>
So do I. Firstly, I'll think how to explain it.

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