  To combine and query ordered data from multiple sources, one needs to handle
uncertainty about the possible orderings. Examples of such "order-incomplete"
data include integrated event sequences such as log entries, lists of
properties (e.g., hotels and restaurants) ranked by an unknown function
reflecting relevance or customer ratings, and documents edited concurrently
with an uncertain order on edits. This paper introduces a query language for
order-incomplete data, based on the positive relational algebra with
order-aware accumulation. We use partial orders to represent order-incomplete
data, and study possible and certain answers for queries in this context. We
show that these problems are respectively NP-complete and coNP-complete, but
identify many tractable cases depending on the query operators or input partial
orders.
