A method, computer system, and computer program are claimed for selecting rows from first and second tables each having rows containing values in columns. In at least the first table, the rows are divided into partitions at least one of which is populated by one or more rows. The method, computer system, and computer program include creating a plurality of partition contexts, each partition context corresponding to a populated partition and storing at least a first value associated with a row. The lowest first value stored by the partition context is determined. Rows with a particular first value are identified by reading the partition contexts, creating a temporary file context corresponding to one of the partition contexts, the temporary file context including location data for a data block of rows in memory, and reading rows in memory.