If we choose nosql database as a cloudant db then why need of warehouse dashdb?
Is this useful to handle large non-sql data and service will connect to ware house instead of cloudant.
Cloudant databases and dashDB warehouses are really complementary technologies. Cloudant is a noSQL cloud database and entirely capable to handle large volumes of data. You can also build analytic applications with the use of map-reduce functions. However, your data will always be presented and persisted in JSON format.
For a large number of applications it is useful to have your data in table representation instead of JSON documents. Data Warehousing technology has matured for 30+ years now where tables are modelled into star schemas using facts and dimensions. Applications use SQL to query those multi-dimensional models for analytics.
We built the integration between Cloudant and dashDB (our data warehouse service in the cloud) to enable those traditional data warehouse applications to work with the JSON documents from Cloudant. This opens up a whole range of solutions you can build with Cloudant. It is like a bridge from your domain of noSQL applications into your domain of relational (SQL) applications.
For more details, go check out our documentation at https://docs.cloudant.com/warehousing.html