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.

Examples:

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

See Also:

Constant Summary

NAME = 
[:leftjoin]

Constants inherited from SPARQL::Algebra::Operator

ARITY

Instance Attribute Summary

Attributes included from Query

#solutions

Attributes inherited from SPARQL::Algebra::Operator

#operands, #options

Instance Method Summary (collapse)

Methods included from Query

#context=, #each_solution, #failed?, #matched?, #unshift, #variables

Methods inherited from SPARQL::Algebra::Operator

arity, base_uri, #base_uri, base_uri=, #boolean, #constant?, #eql?, #evaluatable?, evaluate, #executable?, for, #initialize, #inspect, #operand, prefixes, #prefixes, prefixes=, #to_sse, #to_sxp, #variable?

Methods included from Evaluatable

#apply, #evaluate, #memoize

Methods included from Expression

cast, #constant?, #evaluate, for, new, open, parse, #to_sse, #variable?

Constructor Details

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

Instance Method Details

- (RDF::Query::Solutions) execute(queryable, options = {})

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.

Parameters:

  • queryable (RDF::Queryable) —

    the graph or repository to query

  • options (Hash{Symbol => Object}) (defaults to: {}) —

    any additional keyword options

Returns:

See Also:



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# File 'lib/sparql/algebra/operator/left_join.rb', line 33

def execute(queryable, options = {})
  filter = operand(2)

  
  debug("LeftJoin", options)
  left = operand(0).execute(queryable, options.merge(:depth => options[:depth].to_i + 1)) || {}
  debug("=>(left) #{left.inspect}", options)
  right = operand(1).execute(queryable, options.merge(:depth => options[:depth].to_i + 1)) || {}
  debug("=>(right) #{right.inspect}", options)
  
  # LeftJoin(Ω1, Ω2, expr) =
  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("===>(evaluate) #{s.inspect}", options) 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("=>(merge s1 s2) #{s.inspect}", options)
        solutions << s
        load_left = false   # Left solution added one or more times due to merge
      end
    end
    if load_left
      debug("=>(add) #{s1.inspect}", options)
      solutions << s1
    end
  end
  
  @solutions = RDF::Query::Solutions.new(solutions)
  debug("=> #{@solutions.inspect}", options)
  @solutions
end

- (Union, RDF::Query) optimize

Returns an optimized version of this query.

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

Returns:



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# File 'lib/sparql/algebra/operator/left_join.rb', line 77

def optimize
  ops = operands.map {|o| o.optimize }.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(ops[0], ops[1], expr) : LeftJoin(ops[0], ops[1])
  end
end