[BUG] Subquery in From doesn't work as expected.

[penghuo]

The following query return all the result instead of selected results

POST /_opendistro/_sql/_explain
{
  "query" : """
        SELECT a.firstname AS my_first, a.lastname AS my_last, a.age AS my_age
        FROM (
          SELECT firstname, lastname, age
          FROM accounts
          WHERE age > 30
        ) AS a
        """
}