Class: SPARQL::Algebra::Operator::LeftJoin

Inherits:
SPARQL::Algebra::Operator show all
Includes:
Query
Defined in:
lib/sparql/algebra/operator/left_join.rb

Overview

The SPARQL GraphPattern leftjoin operator.

[57] OptionalGraphPattern ::= 'OPTIONAL' GroupGraphPattern

Examples:

SPARQL Grammar

PREFIX :    <http://example/>
SELECT * { 
  ?x :p ?v .
  OPTIONAL { 
    ?y :q ?w .
    FILTER(?v=2)
  }
}

SSE

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

See Also:

Constant Summary collapse

NAME = 
[:leftjoin]

Constants inherited from SPARQL::Algebra::Operator

ARITY, IsURI, URI

Constants included from Expression

Expression::PATTERN_PARENTS

Instance Attribute Summary

Attributes included from Query

#solutions

Attributes inherited from SPARQL::Algebra::Operator

#operands

Instance Method Summary collapse

Methods included from Query

#each_solution, #empty?, #failed?, #graph_name=, #matched?, #query_yields_boolean?, #query_yields_solutions?, #query_yields_statements?, #unshift, #variables

Methods inherited from SPARQL::Algebra::Operator

#aggregate?, arity, #base_uri, base_uri, base_uri=, #bind, #boolean, #constant?, #deep_dup, #each_descendant, #eql?, #evaluatable?, evaluate, #executable?, #first_ancestor, for, #initialize, #inspect, #mergable?, #ndvars, #node?, #operand, #optimize!, #parent, #parent=, #prefixes, prefixes, prefixes=, #rewrite, #to_binary, to_sparql, #to_sxp, #to_sxp_bin, #variable?, #variables, #vars

Methods included from Expression

cast, #constant?, debug, #evaluate, extension, extension?, extensions, for, #invalid?, new, #node?, open, #optimize!, parse, register_extension, #to_sxp_bin, #valid?, #variable?

Constructor Details

This class inherits a constructor from SPARQL::Algebra::Operator

Instance Method Details

#execute(queryable, **options) {|solution| ... } ⇒ RDF::Query::Solutions

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.

Yields:

  • (solution) —

    each matching solution

Yield Parameters:

Yield Returns:

  • (void) —

    ignored

Raises:

  • (ArgumentError)

See Also:



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# File 'lib/sparql/algebra/operator/left_join.rb', line 49

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), **options.merge(depth: options[:depth].to_i + 1))
  debug(options) {"=>(leftjoin left) #{left.inspect}"}

  right = queryable.query(operand(1), **options.merge(depth: options[:depth].to_i + 1))
  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)
      # Re-bind to bindings, if defined, as they might not be found in solution
      options[:bindings].each_binding do |name, value|
        s[name] = value if filter.variables.include?(name)
      end if options[:bindings] && filter.respond_to?(:variables)

      # See https://github.com/w3c/rdf-tests/pull/83#issuecomment-1324220844 for @afs's discussion of the simplified/not-simplified issue.
      #
      # The difference is when simplification is applied. It matters for OPTIONAL because OPTIONAL { ... FILTER(...) } puts the filter into the LeftJoin expressions. In LeftJoin, the FILTER can see the left-hand-side variables. (SQL: LEFT JOIN ... ON ...)
      # 
      # For OPTIONAL { { ... FILTER(...) } }, the inner part is Join({}, {.... FILTER }).
      # 
      # if simplify happens while coming back up the tree generating algebra operations, it removes the join i.e. the inner of {{ }}, and passes "... FILTER()" to the OPTIONAL. The effect of the extra nesting in {{ }} is lost and it exposes the filter to the OPTIONAL rule.
      # 
      # if simplification happens as a step after the whole algebra is converted, this does not happen. Compiling the OPTIONAL see a join and the filter is not at the top level of the OPTIONAl block and so not handled in the LeftJoin.
      # 
      # Use case:
      # 
      # # Include name if person over 18
      # SELECT *
      # { ?person :age ?age 
      #    OPTIONAL { ?person :name ?name. FILTER(?age > 18) }
      # }
      # Hindsight: a better syntax would be call out if the filter needed access to the LHS.
      # 
      # OPTIONAL FILTER(....) { }
      # 
      # But we are where we are.
      # 
      # (a "no conditions on LeftJoin" approach would mean users having to duplicate parts of their query - possibly quite large parts.)
      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) ⇒ Object

Optimizes this query.

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

FIXME

See Also:



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# File 'lib/sparql/algebra/operator/left_join.rb', line 139

def optimize(**options)
  lhs, rhs, expr = operands.map {|o| o.optimize(**options) }
  expr = nil if expr.respond_to?(:true?) && expr.true?

  if lhs.empty? && rhs.empty?
    RDF::Query.new  # Empty query, expr doesn't matter
  elsif rhs.empty?
    # Expression doesn't matter, just use the first operand
    lhs
  elsif lhs.empty?
    # Result is the filter of the second operand if there is an expression
    # FIXME: doesn't seem to work
    #expr ? Filter.new(expr, rhs) : rhs
    self.dup
  else
    expr ? LeftJoin.new(rhs, lhs, expr) : LeftJoin.new(lhs, rhs)
  end
end

#to_sparql(top_level: true, filter_ops: [], extensions: {}, **options) ⇒ String

Returns a partial SPARQL grammar for this operator.



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# File 'lib/sparql/algebra/operator/left_join.rb', line 169

def to_sparql(top_level: true, filter_ops: [], extensions: {}, **options)
  str = "{\n" + operands[0].to_sparql(top_level: false, extensions: {}, **options)
  str << 
    "\nOPTIONAL {\n" +
    operands[1].to_sparql(top_level: false, extensions: {}, **options)
  case operands[2]
  when SPARQL::Algebra::Operator::Exprlist
    operands[2].operands.each do |op|
      str << "\nFILTER (" + op.to_sparql(**options) + ")"
    end
  when nil
  else
    str << "\nFILTER (" + operands[2].to_sparql(**options) + ")"
  end
  str << "\n}}"
  top_level ? Operator.to_sparql(str, filter_ops: filter_ops, extensions: extensions, **options) : str
end

#validate! ⇒ Object

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



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# File 'lib/sparql/algebra/operator/left_join.rb', line 120

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