hadoop warm up
1. Hadoop move-code-to-data, which is distributed among clusters; while traditional distributed system like SETI@home move-data-to-code
2. Hadoop is usually used to process unstructured data, which makes SQL database not suitable
3. SQL optimized for some legacy applications, and may mismatch contemporary application requirements
4. SQL database scale-up rather than scale-out
5. function programing(mapreduce) vs. declarative queries(SQL)