title: what is the df.coalesce(1) means?
body: <p>I used spark with python so i need to save parquet file included spark result </p>
  
  <pre><code>df
  .coalesce(1)
  .write
  .save(save_path, format='parquet', mode='append'))
  </code></pre>
  
  <p>this is Collect data and save to HDFS as parquet format code</p>
  
  <p>so i am curious that if i ran the spark about 30 parquet files 
  the spark result will be save only one parquet file or not</p>
  
  <p>if someone know that the principal plz teach me 
  also is this code is correct or not for save only one parquet files.</p>
  
  <p>thank you</p>