With data.world's federation capabilities it is possible to join tables from disparate data sources without having to worry about the source data formats. While this is a very powerful functionality, it also allows construction of poorly formed queries that can time out when they are run. We have a query optimizer in process that will fix many issues without you even having to think about them, but in the meantime here are some things you can do with your joins to make your queries more efficient.
Be careful when creating joins to ensure that all applicable join conditions are specified.
Use inner joins rather than outer joins, where possible.
Be wary of joins on calculated conditions, including especially joins which require casting.
When possible, group together joined tables that are virtualized from the same sources.