azure sql - create index, does DB have to be offline? New to Azure SQL. Simple example to ask question.
Table Cars. 
Want to add an non clustered index to Cars table. 
Takes a long time, say 30 min during staging test deploy. 

When upgrading production Azure SQL, can we leave the database online while building a table index? During this time while the table is building, can applications send inserts/updates to this table? 

Or do we need to take the production DB offline for 30 to build this index?
You should absolutely be able to access the table while the index builds. Now, depending on your service tier in Azure, you might be using a lot of your resources to build this index, and that might impact user access, but otherwise, no. The index will build, but access to the table will continue.