Module: Sequel::EmulateOffsetWithRowNumber

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.

[Source]

    # File lib/sequel/adapters/utils/emulate_offset_with_row_number.rb, line 8
 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.

[Source]

    # File lib/sequel/adapters/utils/emulate_offset_with_row_number.rb, line 21
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.

[Source]

    # File lib/sequel/adapters/utils/emulate_offset_with_row_number.rb, line 50
50:     def supports_offsets_in_correlated_subqueries?
51:       false
52:     end

Methods

Public Instance methods