How to view contraints of a cloumn in postgresSql?
PostgreSQL exposes the constraints as well as other schema details through information_schema, so to find all the constraints for the table, query table_constraints, for example:

SELECT constraint_name FROM information_schema.table_constraints
    WHERE table_name='my_table' AND constraint_type='UNIQUE';


To find which columns are involved into the constraints, check constraint_column_usage.