[SPARK-18510][SQL] Follow up to address comments in #15951

## What changes were proposed in this pull request?

This PR addressed the rest comments in #15951.

## How was this patch tested?

Jenkins

Author: Shixiong Zhu <shixiong@databricks.com>

Closes #15997 from zsxwing/SPARK-18510-follow-up.

(cherry picked from commit 223fa218)
Signed-off-by: Tathagata Das <tathagata.das1565@gmail.com>