What kind of exercises to be done for lower back
Discuss with your treating doctor...
Lower back stretching exercise are helpful for back pain and to certain extent nerve root pains (sciatica). However it is important to discuss with the treating if those exercise can be used by the patient. If the pain is severe it is best to rest the back on a flat surface and not do any exercise. If the pain is not that severe, check with the treating doctor before starting back exercises.
In appropriate exercise without knowledge of the treating doctor can worsen the state.
Hope this answered the query. Let me know if you need clarifications,