Impala recently encountered an issue with the metastore (IMPALA-4260 ) where column stats would get dropped when adding a column to a table.
The reason seems to be that Hive does a case sensitive check on the column stats types during an "alter table" and expects the types to be all lower case. This case sensitive check doesn't appear to happen when the stats are set in the first place.
We're solving this on the Impala end by storing types in the metastore as all lower case, but Hive's behavior here is very confusing. It should either always be case sensitive, so that you can't create column stats with types that Hive considers invalid, or it should never be case sensitive.