waynexia opened a new pull request #8662:
URL: https://github.com/apache/arrow/pull/8662


   This PR makes datafusion will output evaluation result of `SELECT` 
expression without `FROM` clause.
   
   prev:
   ```
   > select 1 ;
   0 rows in set. Query took 0 seconds.
   ```
   
   now:
   ```
   > select 1;
   +----------+
   | Int64(1) |
   +----------+
   | 1        |
   +----------+
   1 rows in set. Query took 0 seconds.
   ```
   
   Previously, datafusion will generate an `EmptyExec` as input for outer 
`ProjectionExec`.  `EmptyExec` will return an empty stream when executing. And 
`ProjectionExec` won't evaluate its expression as it gets nothing from its 
"input". So I made a stream with one null value as a placeholder to feed it 
instead of `EmptyExec`.
   
   Signed-off-by: wayne <[email protected]>


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