Window queries for large unstructured data sets

A query engine for an unstructured database satisfies window based queries and analytics by defining a window of documents, and performing analytics on the window using a default value for omitted field. A tabular index containing only values needed for analytics and document ordering defines each window. The tabular index includes all fields from each document that are required to satisfy the query, retrieved on a single pass by the query engine so that multiple fetches to the same document are avoided. Since each document in the window need not contain all the same fields as the other documents, an adapter includes logic for defining a default or placeholder value for a field called for in an analytic computation but nonexistent in a particular document. By retrieving only the computationally relevant fields, and by performing the retrieval only once on each document, the I/O overhead is greatly reduced.