How can i obtain the last row of a table, using the insertion order? (the table is not orderable by any column)
The simple answer is that you can't. I recommend you reconstruct your table, preferably with a serial column.
In theory, well, ROWID would work for you (see IBM Informix Dynamic Server v10 Information Center) but in order for it to work reliably, this places rather a lot of assumptions on how you deal with data in this table (never delete, never update statistics high, never update indexed columns). My advice is to rethink the design of this table and add a serial column, that way you can always get the last row that you just inserted using sqlca.sqlerrd1