Module: Sequel::EmulateOffsetWithReverseAndCount

Included in:
Access::DatasetMethods
Defined in:
lib/sequel/adapters/utils/emulate_offset_with_reverse_and_count.rb

Instance Method Summary collapse

Instance Method Details

#empty?Boolean

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.

Returns:

  • (Boolean)

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

def empty?
  if o = @opts[:offset]
    unlimited.count <= o
  else
    super
  end
end

#select_sqlObject

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] && (match = /\A\$(.*)\z/.match(o.to_s))
    # 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.
    s = match[1].to_sym
    if prepared_arg?(s)
      o = prepared_arg(s)
    end
  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

#supports_offsets_in_correlated_subqueries?Boolean

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.

Returns:

  • (Boolean)

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# File 'lib/sequel/adapters/utils/emulate_offset_with_reverse_and_count.rb', line 65

def supports_offsets_in_correlated_subqueries?
  false
end