I am planning to build a DataWarehouse in MongoDB for the first time. It has been suggested to me that I should use Hadoop for map-reduce in case I need some more complex analyses of the datasets. Having discovered Hive, I liked the idea of doing mapreduces through a language similar with SQL. But my doubt is, can I make HiveQL queries directly into mongodb without needing to build an Hive DW on top of Hadoop? Because in all use cases I found it seems to only work in the data found in the Hadoop HDFS.
You could use MongoDB Connector for Hadoop: http://docs.mongodb.org/ecosystem/tools/hadoop/
Mongo DB on its own has Map-Reduce paradigm too: http://docs.mongodb.org/manual/core/map-reduce/