module Sequel::EmulateOffsetWithReverseAndCount

def default_offset_order

By default, orders by all of the columns in the dataset.
The default order to use for datasets with offsets, if no order is defined. 
def default_offset_order
  clone(:append_sql=>String.new, :offset=>nil).columns
end

def empty?

a column that empty does not select.
By default it would break since the order would be based on
Make empty? work with an offset with an order. 
def empty?
  if o = @opts[:offset]
    unlimited.count <= o
  else
    super
  end
end

def select_sql

to work correctly, and you must select all columns that you are ordering on.
reversed in the subselect. Note that the order needs to be unambiguous
If offset is used, an order must be provided, since it needs to be

a count of the number of rows.
Emulate OFFSET support using reverse order in a subselect, requiring 
def select_sql
  return super if @opts[:sql]
  return super unless o = @opts[:offset]
  order = @opts[:order] || default_offset_order
  if order.nil? || order.empty?
    raise(Error, "#{db.database_type} requires an order be provided if using an offset")
  end
  ds = unlimited
  row_count = @opts[:offset_total_count] || ds.clone(:append_sql=>String.new, :placeholder_literal_null=>true).count
  dsa1 = dataset_alias(1)
  if o.is_a?(Symbol) && @opts[:bind_vars] && /\A\$(.*)\z/ =~ o
    # Handle use of bound variable offsets.  Unfortunately, prepared statement
    # bound variable offsets cannot be handled, since the bound variable value
    # isn't available until later.
    o = prepared_arg($1.to_sym)
  end
  reverse_offset = row_count - o
  ds = if reverse_offset > 0
    ds.limit(reverse_offset).
      reverse(*order).
      from_self(:alias=>dsa1).
      limit(@opts[:limit]).
      order(*order)
  else
    # Sequel doesn't allow a nonpositive limit.  If the offset
    # is greater than the number of rows, the empty result set
    # shuld be returned, so use a condition that is always false.
    ds.where(1=>0)
  end
  sql = @opts[:append_sql] || String.new
  subselect_sql_append(sql, ds)
  sql
end

def supports_offsets_in_correlated_subqueries?

a count that will be invalid if a correlated subquery is used.
This does not support offsets in correlated subqueries, as it requires a query to get 
def supports_offsets_in_correlated_subqueries?
  false
end