New Features

Dataset#avg, interval, min, max, range, and sum now accept virtual row blocks, allowing you to more easily get aggregate values of expressions based on the table:
```
  DB[:table].sum{some_function(column1, column2)} # => 134
  # SELECT sum(some_function(column1, column2)) FROM table
```
Database#do has been added on PostgreSQL for using the DO anonymous code block execution statement.
Model.dataset_module now uses a Module subclass, which allows you to call subset inside a dataset_module block, making it easier to consolidate dataset method code:
```
  class Album < Sequel::Model
    dataset_module do
      subset(:gold){copies_sold > 500000}
    end
  end
```
Database#copy_table and copy_into are now supported on jdbc/postgres.
Sequel now supports deferred constraints on constraint types other than foreign keys. The only databases that appear to implement this are Oracle and PostgreSQL.
Sequel now supports INITIALLY IMMEDIATE deferred constraints via the :deferrable=>:immediate constraint/column option.
Sequel now supports setting the default size of string columns, via the default_string_column_size option or accessor. In some cases, Sequel‘s default string column size of 255 is too large (e.g. MySQL with utf8mb4 character set), and this allows you to change it.

Other Improvements

Dataset#count and other methods now use a subselect in the case where the dataset has an offset but no limit.
If an error occurs while attempting to commit a transaction, Sequel now attempts to rollback the transaction. Some databases do this automatically, but not all. Among other things, this fixes issues with deferred foreign key constraint violations on SQLite.
When extending a model‘s dataset, the model‘s instance_dataset is reset, insuring that it will also be extended with the module.
When passing an invalid argument to Dataset#filter, the exception message now includes the argument.
The force_encoding plugin now works with frozen string values.
Public methods added to a model dataset_module now have model class methods created for them even if the method was added outside of a dataset_module block.
On PostgreSQL, Database#indexes now includes a :deferrable entry for each index hash, which will be true for unique indexes where the underlying constraint is deferrable.
On Microsoft SQL Server 2000, Dataset#update no longer includes a limit (TOP), allowing it to work correctly.

Model.dataset_methods has been removed. This was used to store blocks for methods created via def_dataset_method and subset. The internals have been changed so that a dataset_module is always used in these cases, therefore there was no longer a reason for this method.