Does creating indexes require downtime
If you're on SQL Server Enterprise Edition, you can add the ONLINE switch:

CREATE NONCLUSTERED INDEX IX_Table1_URL
ON [dbo].[Table1] ([URL])
WITH (ONLINE=ON);
GO


Most of the index creation will happen without blocking, although not all - read this DBA.se question &amp; answer for details about when even online index creation blocks other queries.

Your server may still be much slower during the index creation, though - creating indexes can be hard work, taxing your SQL Server's storage.