[GitHub] [arrow-datafusion] ozankabak commented on pull request #6445: Add support for FIRST_VALUE, LAST_VALUE Aggregate Functions

["ozankabak (via GitHub)" <gi...@apache.org>]

ozankabak commented on PR #6445:
URL: https://github.com/apache/arrow-datafusion/pull/6445#issuecomment-1563304302

   Note that this change makes `FIRST_VALUE` and `LAST_VALUE` functions, which are already available as window functions, also available in an aggregation context so that one can choose first/last values within a group according to a given ordering. These functions are sometimes called `FIRST` and `LAST` in other query engines/DBs, but we chose to use the same with their window function counterparts (as in the case of `SUM`, `COUNT` etc.)
   
   Having said that, there are few minor issues here that we will fix shortly. I think the example in the PR body should read something like:
   ```sql
   SELECT a, FIRST_VALUE(c ORDER BY b DESC) as first_c
   FROM table
   GROUP BY a
   ```
   This will return the first `c` value, according to the reverse `b` order, for every `a` group.
   
   Also some tests are failing. We will fix the tests, make the PR body clearer and possibly add another test that looks like this.


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