dear all， Recently when I am reading MAL optimizer manual of MonetDB.and don't know the Merge Table optimizer,A merge association table (MAT). (1)How it perform optimize the MAL? (2) for this example, before optimize, m0:= bat.new(:oid,:int); m1:= bat.new(:oid,:int); m2:= bat.new(:oid,:int); b := mat.new(m0,m1,m2); s := algebra.select(b,1,3); i := aggr.count(s); io.print(s); io.print(i); c0 := bat.new(:int,:int); c1 := bat.new(:int,:int); c := mat.new(c0,c1); j := algebra.join(b,c); io.print(j); The selection and aggregate operations can simply be rewritten using a MAT: _33 := algebra.select(m0,1,3); _34 := algebra.select(m1,1,3); _35 := algebra.select(m2,1,3); s := mat.new(_33,_34,_35); i := 0:int; _36 := aggr.count(_33); i := calc.+(i,_36); Chapter 5: The MAL Optimizer 90 _37 := aggr.count(_34); i := calc.+(i,_37); _38 := aggr.count(_35); i := calc.+(i,_38); io.print(i); It seems that mat is very same with the bat.kunion, but what different between them, and how it optimizer the MAL? Best regards!