Adaptive Server’s default isolation level is 1, which prevents dirty reads. Adaptive Server enforces isolation level 1 by:
Applying exclusive locks on pages or tables being changed. It holds those locks until the end of the transaction. Only a process at isolation level 0 can read a page locked by an exclusive lock.
Applying shared locks on pages being searched. It releases those locks after processing the row, page or table.
Using exclusive and shared locks allows Adaptive Server to maintain the consistency of the results at isolation level 1. Releasing the shared lock after the scan moves off a page improves Adaptive Server’s concurrency by allowing other transactions to get their exclusive locks on the data.