Contents Index Merge sort Recursive union

ASA SQL User's Guide
  Query Optimization and Execution
    Query execution algorithms
      Sorting and unions

Union all

The union all algorithm reads rows from each of its inputs and outputs them, regardless of duplicates. This algorithm is used to implement UNION and UNION ALL clauses. In the UNION case, a duplicate elimination algorithm is needed to remove any duplicates generated by the union all.


Contents Index Merge sort Recursive union