Index Direction

All indexes in Firebird are uni-directional.An index may be constructed from the lowest value to the highest (ascending order) or from the highest value to the lowest (descending order).The keywords ASC[ENDING] and DESC[ENDING] are used to specify the direction of the index.The default index order is ASC[ENDING].It is valid to define both an ascending and a descending index on the same column or key set.

Tip	A descending index can be useful on a column that will be subjected to searches on the high values (“newest”, maximum, etc.)

Firebird uses B-tree indexes, which are bidirectional.However, due to technical limitations, Firebird uses an index in one direction only.

Computed (Expression) Indexes

In creating an index, you can use the COMPUTED BY clause to specify an expression instead of one or more columns.Computed indexes are used in queries where the condition in a WHERE, ORDER BY or GROUP BY clause exactly matches the expression in the index definition.The expression in a computed index may involve several columns in the table.

Note	Expression indexes can also be used as a workaround for indexing computed columns: use the name of the computed column as the expression.