Primary Key...Is One Required?

I have a question regarding primary keys for tables. Is it necessary to have a primary key for a table? According to Informix Documentation, if a primary key does not exist for a table, a primary key is composed of all columns taken together. Does this mean that Informix automatically generates a primary key? I do a database dump (dbschema) and do not see a primary key listed for my table. Also, I recently attended an Informix Class where the instructor said that Informix does not automatically generate a primary key. I'm now wondering whether or not I should create a serial record_num primary key for this table because if one is automatically generated out of every column (I have a large number of columns) and because primary keys are automatically indexed, this will be a very large/inefficient index. Thanks, Bonnie Annelin