1. A method performed by a search engine system, the method comprising: maintaining one or more search engine indexes, associated with the search engine system, that index base items and collections items that includes a collection item; wherein each of the collection items: represents a collection of base items, and is treated, within the one or more search engine indexes, as an entity distinct from the base items represented by the collection item; storing, in a search engine index, of the one or more search engine indexes, a first index entry for a first collection item; wherein the first index entry represents the first collection item itself, rather than any individual base items represented by the first collection item; in response to receiving a search query that is associated with one or more search criteria: searching the one or more search engine indexes for items that satisfy the one or more search criteria, wherein searching the one or more search engine indexes comprises analyzing the first index entry and determining that the first collection item satisfies the one or more search criteria; and identifying a set of one or more matching items that includes the first collection item; responding to said search query by generating, based at least in part on the set of one or more matching items, one or more search results that indicate the first collection item; and wherein the method is performed by one or more computing devices.