0.9.1 is the first maintenance release on 0.9 line and contains over 40 bug fixes and improvements. 

Please see CHANGES.txt for more details: https://git-wip-us.apache.org/repos/asf/tez/repo?p=tez.git;a=blob_plain;f=CHANGES.txt;hb=refs/tags/rel/release-0.9.1
List of issues fixed in 0.9.1: https://issues.apache.org/jira/browse/TEZ/fixforversion/12341127