1. A method for indexing and searching for resources, the method comprising: storing, by a computing system, an index comprising a plurality of index entries, the index entries partitioned among a plurality of index partitions, each index partition in the plurality of index partitions associated with at least one website in a plurality of websites; for each given website in the plurality of websites: automatically updating, by the computing system, the index such that a given index partition in the plurality of index partitions comprises index entries that map terms in a first set of terms to web pages in the given website that contain terms in the first set of terms, the given website associated with the given index partition; automatically obtaining, by the computing system, a set of external partition identifiers, each external partition identifier in the set of external partition identifiers identifying external partitions of external data sources used by the given website; and for each given external partition identifier in the set of external partition identifiers, automatically updating, by the computing system, the index such that the given index partition comprises index entries that map terms in a second set of terms to external resources in the given set of external resources that contain terms in the second set of terms, a given external partition identified by the given external partition identifier containing the given set of external resources; receiving, by the computing system, a query request from a client, the query request specifying a query term, the query request indicating a target website; and in response to receiving the query request: automatically identifying, by the computing system, a target index partition in the plurality of index partitions, the target website associated with the target index partition; automatically using, by the computing system, index entries in the target index partition to identify a relevant set of resources, each resource in the relevant set of resources containing the query term; and providing, by the computing system, a relevant set of resource identifiers to the client, each resource identifier in the relevant set of resource identifiers identifying a resource in the relevant set of resources.