class SPARQL::Algebra::Operator::LeftJoin

The SPARQL GraphPattern `leftjoin` operator.

@example

(prefix ((: <http://example/>))
  (leftjoin
    (bgp (triple ?x :p ?v))
    (bgp (triple ?y :q ?w))
    (= ?v 2)))

@see www.w3.org/TR/sparql11-query/#sparqlAlgebra

Constants

NAME

Public Instance Methods

execute(queryable, **options, &block) click to toggle source

Executes each operand with `queryable` and performs the `leftjoin` operation by adding every solution from the left, merging compatible solutions from the right that match an optional filter.

@param [RDF::Queryable] queryable

the graph or repository to query

@param [Hash{Symbol => Object}] options

any additional keyword options

@yield [solution]

each matching solution

@yieldparam [RDF::Query::Solution] solution @yieldreturn [void] ignored @return [RDF::Query::Solutions]

the resulting solution sequence

@see www.w3.org/TR/sparql11-query/#sparqlAlgebra @see www.rubydoc.info/github/ruby-rdf/rdf/RDF/Query/Solution#merge-instance_method @see www.rubydoc.info/github/ruby-rdf/rdf/RDF/Query/Solution#compatible%3F-instance_method

# File lib/sparql/algebra/operator/left_join.rb, line 37
def execute(queryable, **options, &block)
  filter = operand(2)

  raise ArgumentError,
    "leftjoin operator accepts at most two arguments with an optional filter" if
    operands.length < 2 || operands.length > 3

  debug(options) {"LeftJoin"}
  left = queryable.query(operand(0), depth: options[:depth].to_i + 1, **options)
  debug(options) {"=>(leftjoin left) #{left.inspect}"}

  right = queryable.query(operand(1), depth: options[:depth].to_i + 1, **options)
  debug(options) {"=>(leftjoin right) #{right.inspect}"}

  # LeftJoin(Ω1, Ω2, expr) =
  @solutions = RDF::Query::Solutions()
  left.each do |s1|
    load_left = true
    right.each do |s2|
      s = s2.merge(s1)
      expr = filter ? boolean(filter.evaluate(s)).true? : true rescue false
      debug(options) {"===>(evaluate) #{s.inspect}"} if filter

      if expr && s1.compatible?(s2)
        # { merge(μ1, μ2) | μ1 in Ω1 and μ2 in Ω2, and μ1 and μ2 are compatible and expr(merge(μ1, μ2)) is true }
        debug(options) {"=>(merge s1 s2) #{s.inspect}"}
        @solutions << s
        load_left = false   # Left solution added one or more times due to merge
      end
    end
    if load_left
      debug(options) {"=>(add) #{s1.inspect}"}
      @solutions << s1
    end
  end
  
  debug(options) {"=> #{@solutions.inspect}"}
  @solutions.each(&block) if block_given?
  @solutions
end
optimize!(**options) click to toggle source

Optimizes this query.

If optimize operands, and if the first two operands are both Queries, replace with the unique sum of the query elements

@return [Object] a copy of `self` @see SPARQL::Algebra::Expression#optimize FIXME

# File lib/sparql/algebra/operator/left_join.rb, line 98
def optimize!(**options)
  return self
  ops = operands.map {|o| o.optimize(**options) }.select {|o| o.respond_to?(:empty?) && !o.empty?}
  expr = ops.pop unless ops.last.executable?
  expr = nil if expr.respond_to?(:true?) && expr.true?
  
  # ops now is one or two executable operators
  # expr is a filter expression, which may have been optimized to 'true'
  case ops.length
  when 0
    RDF::Query.new  # Empty query, expr doesn't matter
  when 1
    expr ? Filter.new(expr, ops.first) : ops.first
  else
    expr ? LeftJoin.new(ops[0], ops[1], expr) : LeftJoin.new(ops[0], ops[1])
  end
end
validate!() click to toggle source

The same blank node label cannot be used in two different basic graph patterns in the same query

Calls superclass method SPARQL::Algebra::Operator#validate!
# File lib/sparql/algebra/operator/left_join.rb, line 79
def validate!
  left_nodes, right_nodes = operand(0).ndvars, operand(1).ndvars

  unless (left_nodes.compact & right_nodes.compact).empty?
    raise ArgumentError,
         "sub-operands share non-distinguished variables: #{(left_nodes.compact & right_nodes.compact).to_sse}"
  end
  super
end