change_column_default

# File activerecord/lib/active_record/connection_adapters/postgresql/schema_statements.rb, line 437
        def change_column_default(table_name, column_name, default_or_changes) # :nodoc:
          clear_cache!
          column = column_for(table_name, column_name)
          return unless column

          default = extract_new_default_value(default_or_changes)
          alter_column_query = "ALTER TABLE #{quote_table_name(table_name)} ALTER COLUMN #{quote_column_name(column_name)} %s"
          if default.nil?
            # <tt>DEFAULT NULL</tt> results in the same behavior as <tt>DROP DEFAULT</tt>. However, PostgreSQL will
            # cast the default to the columns type, which leaves us with a default like "default NULL::character varying".
            execute alter_column_query % "DROP DEFAULT"
          else
            execute alter_column_query % "SET DEFAULT #{quote_default_expression(default, column)}"
          end
        end