module Sequel::EmulateOffsetWithRowNumber

  1. lib/sequel/adapters/utils/emulate_offset_with_row_number.rb

Public Instance methods

empty?()

If the offset must be emulated with ROW_NUMBER, don’t remove any ordering, because it can cause invalid queries to be issued if an offset is required when ordering.

[show source] 
   # File lib/sequel/adapters/utils/emulate_offset_with_row_number.rb
 8 def empty?
 9   return super unless emulate_offset_with_row_number?
10   select(Dataset::EMPTY_SELECT).limit(1).single_value!.nil?
11 end
select_sql()

Emulate OFFSET support with the ROW_NUMBER window function

The implementation is ugly, cloning the current dataset and modifying the clone to add a ROW_NUMBER window function (and some other things), then using the modified clone in a subselect which is selected from.

If offset is used, an order must be provided, because the use of ROW_NUMBER requires an order.

[show source] 
   # File lib/sequel/adapters/utils/emulate_offset_with_row_number.rb
21 def select_sql
22   return super unless emulate_offset_with_row_number?
23 
24   offset = @opts[:offset]
25   order = @opts[:order]
26   if require_offset_order?
27     order ||= default_offset_order
28     if order.nil? || order.empty?
29       raise(Error, "#{db.database_type} requires an order be provided if using an offset")
30     end
31   end
32 
33   columns = clone(:append_sql=>String.new, :placeholder_literal_null=>true).columns
34   dsa1 = dataset_alias(1)
35   rn = row_number_column
36   sql = @opts[:append_sql] || String.new
37   subselect_sql_append(sql, unlimited.
38     unordered.
39     select_append(Sequel.function(:ROW_NUMBER).over(:order=>order).as(rn)).
40     from_self(:alias=>dsa1).
41     select(*columns).
42     limit(@opts[:limit]).
43     where(SQL::Identifier.new(rn) > offset).
44     order(rn))
45   sql
46 end
supports_offsets_in_correlated_subqueries?()

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

[show source] 
   # File lib/sequel/adapters/utils/emulate_offset_with_row_number.rb
50 def supports_offsets_in_correlated_subqueries?
51   false
52 end