A database query may include an operator requesting two or more incoming tuple streams be combined into a result tuple stream. Generating a query execution plan may include constructing an equivalence union enumeration lookup table for a result domain of an element within the result tuple stream by taking a set union of incoming tuple domains, wherein each distinct value within that result domain is assigned an enumeration value. Generating the query execution plan may include constructing a secondary enumeration for each incoming tuple stream, wherein each secondary enumeration maps enumerated values within the incoming tuple stream into secondary ordinal values that correspond to equivalence union enumeration values. Generating the query execution plan may include mapping an incoming enumeration value through the secondary enumeration to produce an equivalence union enumeration value, and/or mapping, with the equivalence union enumeration lookup table, the equivalence union enumeration value to a cell value.