ANSI Read Uncommitted and HCL Informix® Dirty Read isolation

The simplest isolation level, ANSI Read Uncommitted and Informix® Dirty Read, amounts to virtually no isolation. When a program fetches a row, it places no locks, and it respects none; it simply copies rows from the database without regard to what other programs are doing.

A program always receives complete rows of data. Even under ANSI Read Uncommitted or Informix® Dirty Read isolation, a program never sees a row in which some columns are updated and some are not. However, a program that uses ANSI Read Uncommitted or Informix® Dirty Read isolation sometimes reads updated rows before the updating program ends its transaction. If the updating program later rolls back its transaction, the reading program processes data that never really existed (possibility number 4 in the list of concurrency issues).