= New Adapter Support

* Sequel now has much better support for the DB2 database.

* An ibmdb adapter has been added, and is the recommended adapter to
  to use if you want to connect to DB2 from MRI.

* A jdbc db2 subadapter has been added, allowing good DB2 support on
  JRuby.

* The db2 adapter has been cleaned up substantially, and now works
  well, but it is still recommended that you switch to ibmdb if you
  are using the db2 adapter.

* The firebird adapter has been split into shared and specific parts,
  and quite a few fixes were made to it.

* A jdbc firebird subadapter has been added, allowing connection to
  firebird databases from JRuby.

= New PostgreSQL 9.1 Features

* Dataset#returning has been added for using the RETURNING clause on
  INSERT/UPDATE/DELETE queries.  RETURNING allows such queries to
  return results in much the same way as a SELECT query works.
  When Dataset#returning is used, Dataset #insert, #update, and
  #delete now accept a block that is passed to Dataset #fetch_rows 
  which is yielded plain ruby hashes for each row inserted, updated,
  or deleted.  If Dataset#returning is used and a block is not given
  to those methods, those methods will return an array of plain hashes
  for all rows inserted, updated, and deleted.

* Dataset#with_sql now treats a symbol as a first argument as a method
  name to call to get the SQL.  The expected use case for this is with
  Dataset#returning and insert/update/delete:
    
    DB[:items].
     returning(:id).
     with_sql(:update_sql, :b => :b + 1).
     map(:id)
    
  Basically, it makes it more easily to statically set the
  insert/update/delete SQL, and then be able to use the full
  dataset API for returning results.  As mentioned above, using
  Dataset#returning with #insert, #update, and #delete yields plain
  hashes, so if you want to have the row_proc applied (e.g. you are
  using models), you need to use this method instead, since you can
  then call #each or #all to make sure the row_proc is called on all
  returned rows.

* Dataset#with (common table expressions) now affects
  INSERT/UPDATE/DELETE queries.

* Database#create_table? now uses CREATE TABLE IF NOT EXISTS on
  PostgreSQL 9.1.

= Other New Features

* The :limit option is now respected when eager loading via either
  eager or eager_graph.  By default, Sequel will just do an array
  slice of the resulting ruby array, which gets the correct answer,
  but does not offer any performance improvements.  Sequel also
  offers a new :eager_limit_strategy option for using more advanced
  query types that only load the related records from the database.
  The available values for the :eager_limit_strategy option are:

    :window_function - This uses the row_number window function
      partitioned by the related key fields.  It can only be used
      on databases that support window functions (PostgreSQL 8.4+,
      Microsoft SQL Server 2005+, DB2).
    :correlated_subquery - This uses a correlated subquery that is
      limited.  It works on most databases except MySQL and DB2.

  You can provide a value of true as the option to have Sequel
  pick a strategy to use. Sequel will never use a correlated
  subquery for true, since in some cases it can perform worse than
  loading all related records and doing the array slice in ruby.

  If you want to enable an eager_limit_strategy globally, you can
  set Sequel::Model.default_eager_limit_strategy to a value, and
  all associations that use :limit will default to using that
  strategy.

* one_to_one associations that do not represent true one-to-one
  database relationships, but represent one-to-many relationships
  where you are only returning the first object based on a given
  order are also now handled correctly when eager loading.
  Previously, eager loading such associations resulted in the last
  matching object being associated instead of the first matching
  object being associated.

  You can also use an :eager_limit_strategy for one_to_one
  associations.  In addition to the :window_function and
  :correlated_subquery values, there is also a :distinct_on value
  that is available on PostgreSQL for using DISTINCT ON, which is
  the fastest strategy if you are using PostgreSQL.

* Dataset#map, #to_hash, #select_map, #select_order_map, and
  #select_hash now accept arrays of symbols, and if given arrays
  of symbols, use arrays of results.  For example:

    DB[:items].map([:id, :name])
    # => [[1, 'foo'], [2, 'bar'], ...]
    DB[:items].to_hash([:id, :foo_id], [:name, :bar_id])
    # => {[1, 3]=>['foo', 5], [2, 4]=>['bar', 6], ...}

* For SQL expression objects where Sequel cannot deduce the type
  of the object, it now will consider the type of the argument
  when a &, |, or + operator is used.  For example:

    :x & 1

  Previously, this did "x AND 1", now it does "x & 1".  Using a
  logical operator on an integer doesn't make sense, but it's
  possible people did so if the database uses 1/0 for true/false.
  Likewise:

    :x + 'foo'

  Previously, this did "x + 'foo'" (addition), now it does
  "x || 'foo'" (string concatenation).

* The sql_string, sql_number, and sql_boolean methods are now
  available on SQL::ComplexExpressions, so you can do:

    (:x + 1).sql_string + ' foos'
    # (x + 1) || ' foos'

  Previously, there was not an easy way to generate such SQL
  expressions.

* :after_load association hooks are now applied when using
  eager_graph.  Previously, they were only applied when using
  eager, not when using eager_graph.

* Database#copy_table has been added to the postgres adapter if pg
  is used as the underlying driver.  It allows you to get very
  fast exports of table data in text or CSV format.  It also
  accepts datasets, allowing fast exports of arbitrary queries
  in text or CSV format.

* SQL extract support (:timestamp.extract(:year)) is now emulated
  on the databases that don't natively support it, such as SQLite,
  Microsoft SQL Server, and DB2.  At least the following values are
  supported for extraction: :year, :month, :day, :hour, :minute,
  and :second.

* The bitwise XOR operator is now emulated on SQLite.  Previously,
  attempting to use it would cause the database to raise an error.

* A Database#use_timestamp_timezones accessor has been added on
  SQLite.  This allows you to turn off the use of timezones in
  timestamps by setting the value to false.  This is necessary if you
  want you want to use the SQLite datetime functions, or the new
  ability to emulate extract.
    
  Note that this setting does not affect the current database
  content.  To convert old databases to the new format, you'll
  have to resave all rows that have timestamps.

  At some point in the future, Sequel may default to not using
  timezones in timestamps by default on SQLite, so if you would
  like to rely on the current behavior, you should set this
  accessor to true now.

* Sequel now works around bugs in MySQL when using a subselect with
  a LIMIT by using a nested subselect.

* Sequel now works around issues in Microsoft SQL Server and DB2 when
  using a subselect with IN/NOT IN that uses the emulated offset
  support.

* The jdbc adapter now returns java.sql.Clob objects as
  Sequel::SQL::Blobs.

* Sequel now considers database clob types as the :blob schema type.

* Sequel::SQLTime.create has been added for more easily creating
  instances:

    Sequel::SQLTime.create(hour, minute, second, usec)

* Dataset#select_all now accepts SQL::AliasedExpression and
  SQL::JoinClause arguments and returns the appropriate
  SQL::ColumnAll value that selects all columns from the related
  table.

* Model.set_dataset now accepts Sequel::LiteralString objects that
  represent table names.  This usage is not encouraged except in
  rare cases such as using a set returning function in PostgreSQL.

* Dataset#supports_cte? now takes an optional argument specifying the
  type of query (:insert, :update, :delete, :select).  It defaults to
  :select.

* Dataset#supports_returning? has been added.  It requires an
  argument specifying the type of query (:insert, :update, or
  :delete).

* Dataset#supports_cte_in_subqueries? has been added for checking
  for support for this ability.  Apparently, only PostgreSQL
  currently supports this.  For other adapters that support CTEs but
  not in subqueries, if a subquery with a CTE is used in a JOIN, the
  CTE is moved from the subquery to the main query.

* Dataset#supports_select_all_and_column has been added for seeing
  if "SELECT *, foo ..." style queries are supported.  This is false
  on DB2, which doesn't allow such queries.  When it is false, using
  select_append on a dataset that doesn't specifically select columns
  will now change the query to do "SELECT table.*, foo ..." instead,
  working around the limitation on DB2.

* Dataset#supports_ordered_distinct_on? has been added.  Currently,
  this is only true on PostgreSQL.  MySQL can emulate DISTINCT ON
  using GROUP BY, but it doesn't respect ORDER BY, so it some
  cases it cannot be used equivalently.

* Dataset#supports_where_true? has been added for checking for support
  of WHERE TRUE (or WHERE 1 if 1 is true).  Not all databases support
  using such a construct, and on the databases that do not, you have
  to use WHERE (1 = 1) or something similar.

= Other Improvements

* Sequel 3.27.0 was negatively affected by a serious bug in
  ActiveSupport's Time.=== that has still not been fixed, which
  broke the literalization of Time objects.  In spite of the bad
  precedent it sets, Sequel now avoids using Time.=== on a
  subclass of Time to work around this ActiveSupport bug.

* Dataset#with_pk now uses a qualified primary key instead of an
  unqualified primary key, which means it can now be used correctly
  after joining to a separate table.

* Association after_load hooks when lazy loading are now called
  after the association has been loaded, which allows them to change
  which records are cached.  This makes the lazy load case more
  similar to the eager load case.

* The metaprogrammatically created methods that implement Sequel's
  DSL support have been made significantly faster by using
  module_eval instead of define_method.

* The type translation in the postgres, mysql, and sqlite adapters
  has been made faster by using Method objects that result in more
  direct processing.

* Typecasting values for time columns from Time values to
  Sequel::SQLTime values now correctly handles fractional seconds on
  ruby 1.9.

= Backwards Compatibility

* Dataset#insert_returning_sql has been changed to a private method
  in the PostgreSQL and Firebird adapters, and it operates
  differently than it did previously.  The private
  #insert_returning_pk_sql and #insert_returning_select_sql methods
  have been removed.

* Dataset#with_pk no longer does some defensive checking for misuse of
  primary keys (e.g. providing a composite key when the model uses
  a single key).  Previously, Sequel would raise an Error
  immediately, now such behavior is undefined, with the most likely
  behavior being the database raising an Error.

* The :alias_association_type_map and :alias_association_name_map
  settings have been removed from the :eager_graph dataset option,
  in favor of just storing the related association reflection.

* The internals of the db2 adapter have changed substantially, if you
  were relying on some of the private methods defined in it, you will
  probably have to modify your code.

* The firebird adapter was substanially modified, specifically parts
  related to insert returning autogenerated primary key values, so if
  you were previously using the adapter you should probably take more
  care than usual when testing your upgrade.  

* The Dataset::WITH_SUPPORTED constant has been removed.

* The Dataset#supports_cte? method now accepts an optional argument.
  If you overrode this method, your overridden method now must
  accept an optional argument.

* If you were previously doing:

    :x & 1

  and wanting "x AND 1", you have to switch to:

    :x.sql_boolean & 1

  Likewise, if you were previously doing:

    :x + 'foo'

  and wanting "x + 'foo'", you need to switch to:

    :x.sql_number + 'foo'

* Sequel no longer does defensive type checking in the SQL expression
  support, as it was often more strict than the database and would
  not allow the creation of expressions that were valid for the
  database.
