Update 2019: Find out how customers improve your query time and scanned data :
As pointed out in the comments, this question is related to “how BigQuery will cope with my data if it is 100 times larger.” When working with traditional databases, an index is the right solution, but BigQuery is different: as data size grows, BigQuery adds more servers to the mix, maintaining performance almost constant.
In other words, as your data grows, you should expect a linear increase in costs, while performance will remain almost constant. Indexes are not needed. And this is one of the main reasons people choose BigQuery for their analytic workloads.
(Of course, it all depends on your specific use case, please check these statements and let us know!)