Share Mode Locking Problem...

I'm new to the Informix world, so pardon any mistaken assumptions I may have (that's my disclaimer)... I wish to run a process which updates a bunch of rows in a table, makes a calculation, and then rollback all the changes in the end (think what-if scenarios). At the same time as playing out the scenario, I need for other users to be able to have read access to the old (already committed) data. So, I issue the following: begin work; lock table table in share mode; update table set colb = #### where cola = "this"; Then, without comitting/rolling back the above, in another session, I issue: select * from table order by cola; Instead of getting the old values for each row, I get: 243: Could not position within a table (informix.test_update). 107: ISAM error: record is locked. My isolation and transaction modes are set to their defaults (non-ansi databases). Informix is Online 7.2 for IRIX. Anyone have any clues as to why this basic locking strategy is failing? Thanks, Chad