Then we have the question on when data (even big-data) can still be handled in a standard relational database and can still be handled by a "standard" approach. There are some guidelines that can help you. Please do note this is a comparison primarily for handling data in a relational database or in Hadoop. This is not for storing data.
|RDBMS||Hadoop / MapReduce|
|Access||Interactive and batch||Batch|
|Structure||Fixed Schema||Unstructured schema|
|Language||SQL||Procedural (Java, C++, Ruby, etc.)|
|Updates||Read and Write||Write ones, read many times|
By taking this into consideration when you are struggling with the question if you need to use a MapReduce approach or a RDBMS approach it might be a little more easy to make your decision.