Message-ID: <eb73d4e1-4235-41b5-90a8-b0196550f62b@gmail.com>
Date: Wed, 12 Feb 2025 09:54:33 +0100
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Subject: Re: explain analyze rows=%.0f
To: Tom Lane <tgl@sss.pgh.pa.us>, Robert Haas <robertmhaas@gmail.com>
Cc: Ilia Evdokimov <ilya.evdokimov@tantorlabs.com>,
 Matheus Alcantara <matheusssilv97@gmail.com>,
 Guillaume Lelarge <guillaume@lelarge.info>,
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 "Gregory Stark (as CFM)" <stark.cfm@gmail.com>,
 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>,
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From: Andrei Lepikhov <lepihov@gmail.com>
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On 12/2/2025 03:46, Tom Lane wrote:
> Robert Haas <robertmhaas@gmail.com> writes:
>> 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.
> 
>> 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.
> 
> Note that that formulation has nothing especially to do with
> parameterized plan nodes.  Any nestloop inner side would end up
> getting shown with fractional rowcounts.  Maybe that's fine.
I partly agree with this approach. Playing around a bit, I couldn't 
invent a case where we have different numbers of tuples without 
parameters. But I can imagine it is possible or may be possible in 
future. So, it is not necessary to tangle fractional output with a 
parameterised node.
I'm unsure about the inner subtree of a JOIN - subplan may refer to the 
upper query and process a different number of tuples for every 
evaluation without any JOIN operator.
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 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

-- 
regards, Andrei Lepikhov