A computing system hosts a plurality of websites, a plurality of external data sources, and a search service. External resources in each of the external data sources are partitioned among a plurality of external partitions. The search service automatically generates an index comprising index entries. The index entries are partitioned among a plurality of index partitions, each associated with one or more of the websites. For each given index partition, the index entries in the given index partition map terms to resources in the given set of resources that contain the terms. The given set of resources includes web pages in a given set of websites associated with the given index partition and external resources in external partitions used by the given set of websites. The search service uses the index entries in given index partition to identify resources in the given set of resources containing query terms.