According to field name I understand that the distribution is quite sparse for normal index on emp_city.
Will be better to create a bitmap index in case that you apply to this table with OLAP queries or no create any index on this field.
If you have an index on emp_city or emp_city is a part of some normal index and you'll upadate any emp_city value:
the index will be updated automatically. (Yes)
the index become invalid? (No, but I suggest you run statistics on index or on wole table)
the leaf row in the index will be deleted and inserted?or update in the leaf row will take place??
This is leaf block (not leaf row). Entire block will be replaced.