title: How to let Dask know that an index is sorted?
body: <p>According to <a href="https://stackoverflow.com/a/41268526/6185156">this</a> answer, Dask dataframes can perform smart indexing if Dask knows that the dataframe's index is sorted.</p>
  
  <p>How do I let Dask know if an index is sorted?</p>
  
  <p>In my specific situation I am doing something like this:</p>
  
  <pre><code>for source is sources:
    # This df has a datetimeindex that I know to be sorted
    pd = load_pandas_df_from_some_source(source)
    dd = dask.dataframe.from_pandas(pd, chunksize=foo)
    dd.to_hdf(some_unique_filename, '/data')
  </code></pre>
  
  <p>However when I do something like this, the indexing is incredibly slow:</p>
  
  <pre><code> dd = dask.dataframe.read_hdf(some_glob, '/data')
   print(dd.loc['2001-1-1':'2001-1-2'])
  </code></pre>
  
  <p>I assume that Dask doesn't know that my dataframe is sorted. How do I let it know?</p>