title: Can join with Cassandra table get pushdown?
body: <p>I'm using structured stream. I need to left join a huge (billions of rows) Cassandra table to know whether the source data in micro-batch is new or existed in terms of id col. If I do something like:</p>
  
  <pre><code>val src = spark.read.cassandraFormat("src", "ks").load().select("id")
  val query= some_dataset
        .join(src, expr("src.id=some_dataset.id"), joinType = "leftOuter")
        .withColumn("flag", expr("case when src.id is null then 0 else 1 end"))
        .writeStream
        .outputMode("update")
        .foreach(...)
        .start
  </code></pre>
  
  <p>Can Cassandra push down the left join and look up with the join col value in source delta? Is there a way to tell whether the pushdown happened or not?</p>
  
  <p>Thanks</p>