Module Sequel::EmulateOffsetWithReverseAndCount
In: lib/sequel/adapters/utils/emulate_offset_with_reverse_and_count.rb

Methods

Public Instance methods

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.

[Source]

    # File lib/sequel/adapters/utils/emulate_offset_with_reverse_and_count.rb, line 6
 6:     def empty?
 7:       if o = @opts[:offset]
 8:         unlimited.count <= o
 9:       else
10:         super
11:       end
12:     end

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.

[Source]

    # File lib/sequel/adapters/utils/emulate_offset_with_reverse_and_count.rb, line 20
20:     def select_sql
21:       return super if @opts[:sql]
22:       return super unless o = @opts[:offset]
23: 
24:       order = @opts[:order] || default_offset_order
25:       if order.nil? || order.empty?
26:         raise(Error, "#{db.database_type} requires an order be provided if using an offset")
27:       end
28: 
29:       ds = unlimited
30:       row_count = @opts[:offset_total_count] || ds.clone(:append_sql=>'', :placeholder_literal_null=>true).count
31:       dsa1 = dataset_alias(1)
32: 
33:       if o.is_a?(Symbol) && @opts[:bind_vars] && (match = Sequel::Dataset::PreparedStatementMethods::PLACEHOLDER_RE.match(o.to_s))
34:         # Handle use of bound variable offsets.  Unfortunately, prepared statement
35:         # bound variable offsets cannot be handled, since the bound variable value
36:         # isn't available until later.
37:         s = match[1].to_sym
38:         if prepared_arg?(s)
39:           o = prepared_arg(s)
40:         end
41:       end
42: 
43:       reverse_offset = row_count - o
44:       ds = if reverse_offset > 0
45:         ds.limit(reverse_offset).
46:           reverse_order(*order).
47:           from_self(:alias=>dsa1).
48:           limit(@opts[:limit]).
49:           order(*order)
50:       else
51:         # Sequel doesn't allow a nonpositive limit.  If the offset
52:         # is greater than the number of rows, the empty result set
53:         # shuld be returned, so use a condition that is always false.
54:         ds.where(1=>0)
55:       end
56:       sql = @opts[:append_sql] || ''
57:       subselect_sql_append(sql, ds)
58:       sql
59:     end

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.

[Source]

    # File lib/sequel/adapters/utils/emulate_offset_with_reverse_and_count.rb, line 63
63:     def supports_offsets_in_correlated_subqueries?
64:       false
65:     end

[Validate]