Support multiple-column index keys

The key descriptor contains information about an index key. If the index contains more than one key column, the access method might provide for following operator-class considerations:

• The index might require multiple operator classes.

  Each key column corresponds to an operator class.

• The operator class for a particular key column determines the number and names of support functions for that single key column.
• The operator class determines the number and name of strategy functions for the single key column.