[GitHub] [arrow-datafusion] realno commented on pull request #1525: Add stddev operator

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realno commented on pull request #1525:
URL: 
https://github.com/apache/arrow-datafusion/pull/1525#issuecomment-1007822391


   > This is a really nice piece of work @realno - thank you so much ❤️
   > 
   > I especially like the thorough testing.
   > 
   > I did not review the referenced papers, but I did run some basic tests 
against postgres and I got some strange results
   > 
   > ```shell
   > cargo run datafusion-cli
   > ```
   > 
   > Sometimes the answers are different from query to query
   > 
   > ```
   > ❯ select stddev(sq.column1) from (values (1.1), (2.0), (3.0)) as sq;
   > +--------------------+
   > | STDDEV(sq.column1) |
   > +--------------------+
   > | 0.7760297817881877 |
   > +--------------------+
   > 1 row in set. Query took 0.008 seconds.
   > ❯ select stddev(sq.column1) from (values (1.1), (2.0), (3.0)) as sq;
   > +--------------------+
   > | STDDEV(sq.column1) |
   > +--------------------+
   > | NaN                |
   > +--------------------+
   > ```
   > 
   > And neither of those answers matches postgres:
   > 
   > ```
   > alamb=# select stddev(sq.column1) from (values (1.1), (2.0), (3.0)) as sq;
   >          stddev         
   > ------------------------
   >  0.95043849529221686157
   > (1 row)
   > ```
   > 
   > Postgres and google sheets do match <img alt="Screen Shot 2022-01-07 at 4 
15 53 PM" width="368" 
src="https://user-images.githubusercontent.com/490673/148608489-9e64e81c-a627-4991-b3b5-079f19030c16.png";>
   
   


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