Abstract: Methods, systems, and computer programs encoded on computer storage media, for processing user-defined functions in containers One example system receives a query written in a primary query language that calls a user-defined function. The system assigns primary segment hosts to process portions of the query written in the primary query language and assigns first containers to process the call to the user-defined function. The primary segment hosts provide to the first containers the user-defined function and one or more arguments in the call to the user-defined function. The first containers execute the user-defined function. The primary segment hosts receive an initial result of the call to the user-defined function computed by the first containers. The primary segment hosts compute a final result of the query using the initial result. The primary segment hosts provide the final result of the query in response to receiving the query.