sysixcol system table

Each row in the sysixcol system table describes one column of an index listed in sysindex.

Column name	Column type	Description
column_id	UNSIGNED INT	A unique identifier for the column being indexed.
index_id	UNSIGNED INT	A unique identifier for the index that this index-column belongs to.
order	CHAR(1)	Indicates whether the column in the index is kept in ascending (A) or descending (D) order.
sequence	SMALLINT	The order of the column in the index.
table_id	UNSIGNED INT	A unique identifier for the table to which the index applies.

PRIMARY KEY( table_id, index_id, sequence )

FOREIGN KEY( table_id, index_id ) REFERENCES sysindex( table_id, object_id )

FOREIGN KEY( table_id, column_id ) REFERENCES syscolumn( table_id, object_id )