title: pyarrow error: toPandas attempted Arrow optimization
body: <p>when I set pyarrow to true we using spark session, but when I run toPandas(), it throws the error:</p>
  
  <pre><code>"toPandas attempted Arrow optimization because 'spark.sql.execution.arrow.enabled' is set to true. Please set it to false to disable this"
  </code></pre>
  
  <p>May I know why it happens?</p>