When add index in SQL. Error: Timeout expired. The timeout period elapsed prior to completion of the operation or the server is

1. Tools -> Options -> Designers -> Override connection string time-out value for table designer updates:

Change it to a big number like 3000 seconds and change back when update succeed. 

If you uncheck this option, you need reconnect and it effect. So change to big number is best solution.

2. Check connection timeout 

   2.1 When connect, choose options

   2.2 Tools-> Options -> Query Execution

 

3. Check database timeout