# From https://arxiv.org/abs/1210.0481

digraph G {
    rankdir=LR;
    
    subgraph cluster_relations {
        R [group="R"];
        S [group="S"];
        T [group="T"];
        label = "relations";
    }

    subgraph joins {
        #rankdir = TB;
    	subgraph cluster_a {
    	    R_a [label="R(a,_)",group="R"];
            T_a [label="T(a,_)",group="T"];
            join_a[label="⨝a"];
    
            R -> R_a -> join_a;
    		T -> T_a -> join_a;
    		label = "var a";
    	}
    
    	subgraph cluster_b {
    	    S_b [label="S(b,_)",group="S"];
            Ra_b [label="Ra(b)",group="R"];
            join_b[label="⨝b"];
        	join_a -> Ra_b;
    
    		S -> S_b -> join_b;
    		R -> Ra_b -> join_b;
    		label = "var b";
    	}
    	
    	subgraph cluster_c {
            Sb_c [label="Sb(c)",group="S"];
            Ta_c [label="Ta(c)",group="T"];
            join_c[label="⨝c"];
    	    join_a -> Ta_c;
    	    join_b -> Sb_c;
    
    		S -> Sb_c -> join_c;
    		T -> Ta_c -> join_c;
    		label = "var c";
    	}
    	
        join_a -> result;
        join_b -> result;
        join_c -> result;
    }

}