[GitHub] [arrow-datafusion] ovr commented on a change in pull request #1660: fix: substr - correct behaivour with negative start pos

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

ovr commented on a change in pull request #1660:
URL: https://github.com/apache/arrow-datafusion/pull/1660#discussion_r790733421



##########
File path: datafusion/src/physical_plan/functions.rs
##########
@@ -3680,6 +3704,60 @@ mod tests {
             StringArray
         );
         #[cfg(feature = "unicode_expressions")]
+        test_function!(

Review comment:
       You are right, i did another iteration and looks like I found a way how to get similar behaviour
   
   PG:
   
   
   ```
   select substr('alphabet', 0, 5) as s1, substr('alphabet', -5, 10) as s2, substr('alphabet', -5, 4) as s3;
   
   [
     {
       "s1": "alph",
       "s2": "alph",
       "s3": ""
     }
   ]
   ```
   
   DF:
   
   ```
   ❯ select substr('alphabet', 0, 5) as s1, substr('alphabet', -5, 10) as s2, substr('alphabet', -5, 4) as s3;
   +------+------+----+
   | s1   | s2   | s3 |
   +------+------+----+
   | alph | alph |    |
   +------+------+----+
   1 row in set. Query took 0.016 seconds.
   
   ```




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