I recently ran into the mention of INT8 and, by association, SERIAL8 by Informix engineers and a recent redbook. I want to make a quick comment on that.
These two types were added a long time ago to support the eight-byte integers (64 bits). They are defined as being a 10-byte structure that includes two "standard" integers. It was done this was so eight-byte integers could be supported on 32-bit operating systems. Now it appears that most operating systems support a native 64-bit integer. For this reason, new data types were added to Informix version 11.50 (fixpack 1). The new types, BIGINT and BIGSERIAL, take less space and perform better. Here is what the release notice says:
Improved Query Performance for Large Integers and Serial Data
The BIGINT and BIGSERIAL data types, which are provided as alternatives to the INT8 and SERIAL8 data types. can provide better performance than the INT8 and SERIAL8 data types.
So, let's forget about INT8 and SERIAL8 and let's use BIGINT and BIGSERIAL available in Informix 11.50.