= New Features

* A duplicate_columns_handler extension has been added, for printing a
  warning or raising an exception if a dataset returns multiple
  columns with the same name.  You can set this globally for the
  Database:

    DB.extension :duplicate_columns_handler
    DB.opts[:on_duplicate_columns] = :warn
    DB.opts[:on_duplicate_columns] = proc do |columns|
      columns.include?(:foo) ? :raise : :ignore
    end

  or for specific datasets:

    ds = DB[:table].extension(:duplicate_columns_handler)
    ds = ds.on_duplicate_columns(:raise)
    ds = ds.on_duplicate_columns do |columns|
      columns.include?(:foo) ? :raise : :ignore
    end

  This makes it easier to detect when duplicate columns are returned,
  which in some cases can cause undesired behavior, such as the values
  for later columns of the same name overwriting values for earlier
  columns.

* The Dataset#to_hash, #to_hash_groups, #select_hash, and
  #select_hash_groups methods now take an options hash as a third
  argument.  This options hash can now contain a :hash option, which
  specifies the object in which the resulting values should be
  placed.  You can use this to have the values inserted into a
  custom hash, or another object responding to #[] and #[]=.

* A validators_operator validation has been added to the
  validation_helpers plugin:

    class Example < Sequel::Model
      def validate
        super
        validates_operator(:>, 3, :column1)
        validates_operator(:<=, 4, [:column2, :column3])
      end
    end

* The pg_range extension now adds a #register_range_type Database
  method, supporting per-Database custom range types:

    DB.register_range_type('timerange')

* The dataset_associations plugin now supports a
  :dataset_associations_join association option on associations that
  use joined datasets.  This option will have the datasets returned
  by the dataset association methods join to the same tables that
  would be joined when retriving the associated objects, allowing
  selected columns, orders, and filters that reference columns in
  the joined tables to work correctly.

* The Database :preconnect option can now be set to :concurrently,
  which will create the connections in separate threads. This can
  significantly speed up preconnection in high-latency environments.

* The Database :name option is now supported, holding an arbitrary
  name for the database.  Currently, it is only used in PoolTimeout
  exception messages, but it may be used in other places in the
  future.

= Other Improvements

* The prepared_statements_safe plugin now works correctly when using
  CURRENT_DATE and CURRENT_TIMESTAMP default values for columns.

* Sequel now recognizes an addition unique constraint violation on
  Microsoft SQL Server.

* PoolTimeout exception messages now include the server/shard to which
  the connection was attempted when using the sharded threaded
  connection pool.

= Backwards Compatibility

* Users of sequel_pg should upgrade to 1.6.17, as older versions of
  sequel_pg may not work with Sequel 4.34.0+.

* Any custom extensions that override Dataset#to_hash,
  #to_hash_groups, #select_hash, and #select_hash_groups need to
  be modified to add support for accepting the options hash.