Message-ID: <4f7096bc-6b58-492b-8fe9-be54cebfb905@postgrespro.ru>
Date: Tue, 11 Feb 2025 22:43:30 +0300
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Subject: Re: explain analyze rows=%.0f
To: Ilia Evdokimov <ilya.evdokimov@tantorlabs.com>
Cc: Robert Haas <robertmhaas@gmail.com>, Andrei Lepikhov <lepihov@gmail.com>,
 Matheus Alcantara <matheusssilv97@gmail.com>,
 Guillaume Lelarge <guillaume@lelarge.info>,
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 "Gregory Stark (as CFM)" <stark.cfm@gmail.com>, Tom Lane
 <tgl@sss.pgh.pa.us>, Amit Kapila <amit.kapila16@gmail.com>,
 Justin Pryzby <pryzby@telsasoft.com>, Peter Geoghegan <pg@bowt.ie>,
 vignesh C <vignesh21@gmail.com>,
 "David G. Johnston" <david.g.johnston@gmail.com>,
 pgsql-hackers@lists.postgresql.org
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From: Alena Rybakina <a.rybakina@postgrespro.ru>
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Hi! Thank you for your valuable work on this!

On 11.02.2025 22:18, Ilia Evdokimov wrote:
>
> On 11.02.2025 20:41, Robert Haas wrote:
>> On Tue, Feb 11, 2025 at 12:14 PM Andrei Lepikhov <lepihov@gmail.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.
>
I looked at the patch and agree with them. I would suggest adding a 
description of how this can help in analyzing the query plans -
I think there is a lack of a description of the reason why this is done 
in the commit message.
I would also add the same to the documentation with an example.

-- 
Regards,
Alena Rybakina
Postgres Professional