This is a
HCATALOG-527 caused regression, wherein the HCatLoader's way of calling HCatInputFormat causes it to do 2x partition lookups - once without the filter, and then again with the filter.
For tables with large number partitions (100000, say), the non-filter lookup proves fatal both to the client ("Read timed out" errors from ThriftMetaStoreClient cause the server doesn't respond) and to the server (too much data loaded into the cache, OOME, or slowdown).
The fix would be to use a single call that also passes a partition filter information, as was in the case of HCatalog 0.4 sources before
(HCatalog-release-wise, this affects all 0.5.x users)