Using a Column or Column List as the Index Key

These restrictions apply to a column or column list specified as the index key:

• All the columns must exist and must be in the table being indexed.
• The maximum number of columns and total width of all columns depends on the database server. See Creating Composite Indexes.
• You cannot add an ascending index to a column or column list that already has a unique constraint on it. See Using the ASC and DESC Sort-Order Options.
• You cannot add a unique index to a column or to a column list that has a primary-key constraint. The reason is that defining the column or column list as the primary key causes the database server to create a unique internal index on the column or column list; you cannot create another unique index on this column or column list with the CREATE INDEX statement.
• The number of indexes that you can create on the same column or the same set of columns is restricted. See Restrictions on the Number of Indexes on a Set of Columns.

In Dynamic Server, these additional restrictions apply to indexes:

• You cannot create an index on a column of an external table.
• The column cannot be of a collection data type.