Method: Sequel::EmulateOffsetWithReverseAndCount#select_sql

Defined in:
lib/sequel/adapters/utils/emulate_offset_with_reverse_and_count.rb

#select_sql ⇒ Object

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.

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# 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] && /\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