DOC: updated cookbook.rst

Author: jreback

doc/source/cookbook.rst

@@ -519,9 +519,12 @@ csv file and creating a store by chunks, with date parsing as well.
 `Reading in a sequence of files, then providing a global unique index to a store while appending
 <http://stackoverflow.com/questions/16997048/how-does-one-append-large-amounts-of-data-to-a-pandas-hdfstore-and-get-a-natural>`__
 
-`Groupby on a HDFStore
+`Groupby on a HDFStore with low group density
 <http://stackoverflow.com/questions/15798209/pandas-group-by-query-on-large-data-in-hdfstore>`__
 
+`Groupby on a HDFStore with high group density
+<http://stackoverflow.com/questions/25459982/trouble-with-grouby-on-millions-of-keys-on-a-chunked-file-in-python-pandas/25471765#25471765>`__
+
 `Hierarchical queries on a HDFStore
 <http://stackoverflow.com/questions/22777284/improve-query-performance-from-a-large-hdfstore-table-with-pandas/22820780#22820780>`__