Index = Base Table Structure Option
The INDEX = BASE TABLE STRUCTURE option indicates that the base table structure of the constrained table be used for integrity enforcement, rather than a newly created secondary index. The base table structure must not be heap and must match the columns in the constraint definition. Because only non-heap base table structures can be specified using the MODIFY statement (after the table has been created), WITH INDEX = BASE TABLE STRUCTURE can be used only for table constraints defined with the ALTER TABLE (rather than the CREATE TABLE) statement.
The ALTER TABLE statement, which adds the constraint, must be preceded by the WITH INDEX = BASE TABLE STRUCTURE statement.
For example:
ALTER TABLE table_name ADD CONSTRAINT constraint_name
PRIMARY KEY(column(s))
WITH (INDEX = BASE TABLE STRUCTURE)
which indicates that the uniqueness semantics enforced by the index are consistent with Ingres and ANSI rules.