18. The non-transitory computer-readable volatile or non-volatile storage medium of claim 17 , wherein the set of one or more matching items includes one or more particular base items and the first collection item.
17. A non-transitory computer-readable volatile or non-volatile storage medium storing instructions which, when executed by one or more processors, cause the one or more processors to perform: maintaining one or more search engine indexes, associated with a search engine system, each of which indexes base items and collection items that includes a first 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 said each collection item; storing, in a search engine index, of the one or more search engine indexes, a first index entry for the 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 the 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.