[GitHub] [arrow-datafusion] jonmmease opened a new issue, #4843: Match Postgres for stddev and variance on less than 3 values

[GitBox <gi...@apache.org>]

jonmmease opened a new issue, #4843:
URL: https://github.com/apache/arrow-datafusion/issues/4843

   **Describe the bug**
   The behavior of the stddev and variance aggregation differs from Postgres for less than three values.  DataFusion crashes in these cases, whereas Postgres computes a variance for an array with two values and returns `NULL` for an array of one value.
   
   In DataFusion CLI:
   ```sql
   ❯  select stddev(sq.column1) from (values (1.0), (2.0), (3.0)) as sq;
   +--------------------+
   | STDDEV(sq.column1) |
   +--------------------+
   | 1                  |
   +--------------------+
   
   ❯  select stddev(sq.column1) from (values (1.0), (2.0)) as sq;
   ArrowError(ExternalError(Internal("At least two values are needed to calculate variance")))
   
   ❯  select stddev(sq.column1) from (values (1.0)) as sq;
   ArrowError(ExternalError(Internal("At least two values are needed to calculate variance")))
   ```
   
   In Postgres
   ```sql
   ❯  select stddev(sq.column1) from (values (1.0), (2.0), (3.0)) as sq;
   1
   
   ❯ select stddev(sq.column1) from (values (1.0), (2.0)) as sq;
   0.7071067811865475244
   
   ❯ select stddev(sq.column1) from (values (1.0)) as sq;
   NULL
   ```
   
   I would like to suggest DataFusion match the behavior of Postgres. Otherwise every usage of the variance function risks raising an error (depending on the distribution of the groupby column).
   
   **Additional context**
   Variance was first added by @realno in https://github.com/apache/arrow-datafusion/pull/1525.
   


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[GitHub] [arrow-datafusion] ozankabak commented on issue #4843: Match Postgres for stddev and variance on less than 3 values

[GitBox <gi...@apache.org>]

ozankabak commented on issue #4843:
URL: https://github.com/apache/arrow-datafusion/issues/4843#issuecomment-1374595943

   We should definitely calculate something for the two-sample case -- erroring simply doesn't make sense in that case. The second example has various behaviors going around, see the discussion in [Snowflake docs](https://docs.snowflake.com/en/sql-reference/functions/stddev.html). I agree with your sentiment that we should align with Postgres.


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[GitHub] [arrow-datafusion] alamb closed issue #4843: Match Postgres for stddev and variance on less than 3 values

[GitBox <gi...@apache.org>]

alamb closed issue #4843: Match Postgres for stddev and variance on less than 3 values
URL: https://github.com/apache/arrow-datafusion/issues/4843


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