When a leaf node of a subtree is a table scan, the optimizer checks whether any indexes that exist on the table match the sort columns. If so, it analyzes the cost of each possible index bracket scan and compares the least of those with the sum of the cost of the table scan and sort operation.

If the analysis shows an index bracket scan as having less cost than the table scan and sort operation, the optimizer converts the table scan to the index bracket scan and removes the sort node.