The total size of all the columns listed in a UNIQUE, PRIMARY KEY, or FOREIGN KEY clause is limited. The limit is the same as the restriction on the total size of all columns in a composite index. IBM Informix Dynamic Server allows 16 key parts and a width of 390 bytes on a 2K page platform. On 4K page platforms or using a non-default page size allows greater than 390 bytes for the width of indexed columns. IBM Informix SE supports 8 columns and a width of 126 bytes. Other Informix database servers allow 16 columns and 255 bytes. The limit depends on the database server in use, but all servers support a total of at least 120 bytes.
For additional information, see the CREATE TABLE statement in the IBM Informix Guide to SQL: Syntax.