add DB schema documentation

[aronwc]

No description provided.

[Thrimbda]

please DO THIS... I cannot find collection xmatrix anywhere. in your code, there are several update sql and a remove sql, But how did you produce the collection at the first time?

[nkravi]

As MongoDB is schema-less I have used it Just to get/set data. You can reproduce the code and you don't need to have any schema in place.

This would help you to understand it better @Macsnow14 (http://blog.mongodb.org/post/119945109/why-schemaless)

[Thrimbda]

@nkravi I see. but I still can't figure out how did you produce the collection XMATRIX... would you please help me understand that? many thanks.

[Thrimbda]

@nkravi Sorry, It's my fault....

I read the code again carefully. then I realize that it's all good.
except some tiny problem of the package you are using or the website of Quantcast.com.
thank you again!