change_column_null

# File activerecord/lib/active_record/connection_adapters/postgresql/schema_statements.rb, line 443
        def change_column_null(table_name, column_name, null, default = nil) # :nodoc:
          validate_change_column_null_argument!(null)

          clear_cache!
          unless null || default.nil?
            column = column_for(table_name, column_name)
            execute "UPDATE #{quote_table_name(table_name)} SET #{quote_column_name(column_name)}=#{quote_default_expression(default, column)} WHERE #{quote_column_name(column_name)} IS NULL" if column
          end
          execute "ALTER TABLE #{quote_table_name(table_name)} ALTER COLUMN #{quote_column_name(column_name)} #{null ? 'DROP' : 'SET'} NOT NULL"
        end