title: Spark save(write) parquet only one file
body: <p>if i write</p>
  
  <pre><code>dataFrame.write.format("parquet").mode("append").save("temp.parquet")
  </code></pre>
  
  <p>in temp.parquet folder 
  i got the same file numbers as the row numbers </p>
  
  <p>i think i'm not fully understand about parquet but is it natural?</p>