module Sequel::EmulateOffsetWithReverseAndCount

Public Instance Methods

empty?() click to toggle source

Make empty? work with an offset with an order. By default it would break since the order would be based on a column that empty does not select.

Calls superclass method
# File lib/sequel/adapters/utils/emulate_offset_with_reverse_and_count.rb, line 8
def empty?
  if o = @opts[:offset]
    unlimited.count <= o
  else
    super
  end
end
select_sql() click to toggle source

Emulate OFFSET support using reverse order in a subselect, requiring a count of the number of rows.

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

Calls superclass method
# File lib/sequel/adapters/utils/emulate_offset_with_reverse_and_count.rb, line 22
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] && (match = Sequel::Dataset::PreparedStatementMethods::PLACEHOLDER_RE.match(o.to_s))
    # 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.
    s = match[1].to_sym
    if prepared_arg?(s)
      o = prepared_arg(s)
    end
  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
supports_offsets_in_correlated_subqueries?() click to toggle source

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

# File lib/sequel/adapters/utils/emulate_offset_with_reverse_and_count.rb, line 65
def supports_offsets_in_correlated_subqueries?
  false
end

Private Instance Methods

default_offset_order() click to toggle source

The default order to use for datasets with offsets, if no order is defined. By default, orders by all of the columns in the dataset.

# File lib/sequel/adapters/utils/emulate_offset_with_reverse_and_count.rb, line 73
def default_offset_order
  clone(:append_sql=>String.new, :offset=>nil).columns
end