Horton

Re: explain analyze rows=%.0f

Andrei Lepikhov <a.lepikhov@postgrespro.ru>

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From: Andrey Lepikhov <a.lepikhov@postgrespro.ru>
To: Amit Kapila <amit.kapila16@gmail.com>, Justin Pryzby <pryzby@telsasoft.com>
Cc: Peter Geoghegan <pg@bowt.ie>, Robert Haas <robertmhaas@gmail.com>, Greg Stark <stark@mit.edu>, vignesh C <vignesh21@gmail.com>, Ibrar Ahmed <ibrar.ahmad@gmail.com>, "David G. Johnston" <david.g.johnston@gmail.com>, pgsql-hackers <pgsql-hackers@postgresql.org>, Simon Riggs <simon@2ndquadrant.com>, Ron Mayer <rm_pg@cheapcomplexdevices.com>, Euler Taveira de Oliveira <euler@timbira.com>, Tom Lane <tgl@sss.pgh.pa.us>
Date: 2022-11-07T04:20:59Z
Lists: pgsql-hackers 
On 22/7/2022 16:47, Amit Kapila wrote:
> I feel the discussion has slightly deviated which makes it unclear
> whether this patch is required or not?

After quick review I want to express my thoughts.
At first, We have been waiting for this feature for years. Often clients 
give an explain to us where we see something like:
"rows=0, loops=1000000".
Without verbose mode, I can't even understand whether this node produces 
any rows or not.
So, I think this feature is useful for parameterized plans mostly.
Also, printing two decimal digits or even three isn't meaningful - 
sometimes we have a plan where number of loops is about 1E6 or even 1E7, 
but number of real rows is equal 100 or 1000.
To overcome this issue, I see two options:
1. Show the exact number of tuples without division by loops (fair case 
but invasive and bad for automation tools).
2. Show rows in scientific format like X.XXEXX.
I vote for second option.

-- 
regards,
Andrey Lepikhov
Postgres Professional

Commits

  1. EXPLAIN: Always use two fractional digits for row counts.

  2. Adjust EXPLAIN test case to filter out "Actual Rows" values.

  3. Allow EXPLAIN to indicate fractional rows.

  4. Fix pgbench performance issue induced by commit af35fe501.