title: pandas - merge on sliced table gives bigger result
body: <p>I've got to Dataframes (~1000 rows each) with overlapping unicode indexing which I want to merge based on the index.</p>
  
  <p>The weird part is I'm getting fewer results when merging the whole dataframes than I do when I merge slices of them.</p>
  
  <pre><code>In [5]: len(pd.merge(df1,df2,left_index=True,right_index=True))
  Out[5]: 1
  
  In [6]: len(pd.merge(df1[:100],df2[:100],left_index=True,right_index=True))
  Out[6]: 67
  </code></pre>
  
  <p>Does anyone have an explanation? can you think of a "pandas solution" (as opposed to going over the indexes with a for loop)?</p>