Sorts for unindexed or non-leading columns

For unindexed columns and columns that are not the leading columns in indexes, Adaptive Server performs a serial table scan, copying the column values into a worktable. It then sorts the worktable to build the histogram. The sort is performed in serial, unless the with consumers clause is specified.

See Chapter 9, “Parallel Sorting” in Performance and Tuning: Optimizer and Abstract Plans for information on parallel sort configuration requirements.