question: protected TupleExpr selectNextTupleExpr ( List < TupleExpr > expressions , Map < TupleExpr , Double > cardinalityMap , Map < TupleExpr , List < Var > > varsMap , Map < Var , Integer > varFreqMap , Set < String > boundVars ) { TupleExpr result = null ; if ( expressions . size ( ) > 1 ) { double lowestCardinality = Double . POSITIVE_INFINITY ; for ( TupleExpr tupleExpr : expressions ) { double cardinality = getTupleExprCardinality ( tupleExpr , cardinalityMap , varsMap , varFreqMap , boundVars ) ; if ( cardinality < lowestCardinality || result == null ) { lowestCardinality = cardinality ; result = tupleExpr ; } } } else { result = expressions . get ( 0 ) ; } return result ; }
target:  Selects from a list of tuple expressions the next tuple expression that should be evaluated. This method selects the tuple expression with highest number of bound variables, preferring variables that have been bound in other tuple expressions over variables with a fixed value.