Module | Sequel::EmulateOffsetWithReverseAndCount |
In: |
lib/sequel/adapters/utils/emulate_offset_with_reverse_and_count.rb
|
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.
# File lib/sequel/adapters/utils/emulate_offset_with_reverse_and_count.rb, line 8 8: def empty? 9: if o = @opts[:offset] 10: unlimited.count <= o 11: else 12: super 13: end 14: 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.
# File lib/sequel/adapters/utils/emulate_offset_with_reverse_and_count.rb, line 22 22: def select_sql 23: return super if @opts[:sql] 24: return super unless o = @opts[:offset] 25: 26: order = @opts[:order] || default_offset_order 27: if order.nil? || order.empty? 28: raise(Error, "#{db.database_type} requires an order be provided if using an offset") 29: end 30: 31: ds = unlimited 32: row_count = @opts[:offset_total_count] || ds.clone(:append_sql=>String.new, :placeholder_literal_null=>true).count 33: dsa1 = dataset_alias(1) 34: 35: if o.is_a?(Symbol) && @opts[:bind_vars] && (match = Sequel::Dataset::PreparedStatementMethods::PLACEHOLDER_RE.match(o.to_s)) 36: # Handle use of bound variable offsets. Unfortunately, prepared statement 37: # bound variable offsets cannot be handled, since the bound variable value 38: # isn't available until later. 39: s = match[1].to_sym 40: if prepared_arg?(s) 41: o = prepared_arg(s) 42: end 43: end 44: 45: reverse_offset = row_count - o 46: ds = if reverse_offset > 0 47: ds.limit(reverse_offset). 48: reverse_order(*order). 49: from_self(:alias=>dsa1). 50: limit(@opts[:limit]). 51: order(*order) 52: else 53: # Sequel doesn't allow a nonpositive limit. If the offset 54: # is greater than the number of rows, the empty result set 55: # shuld be returned, so use a condition that is always false. 56: ds.where(1=>0) 57: end 58: sql = @opts[:append_sql] || String.new 59: subselect_sql_append(sql, ds) 60: sql 61: 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.
# File lib/sequel/adapters/utils/emulate_offset_with_reverse_and_count.rb, line 65 65: def supports_offsets_in_correlated_subqueries? 66: false 67: end