CREATE TABLE t1(a,b,c);
CREATE INDEX t1x1 ON t1(a,b);
INSERT INTO t1 VALUES(1,NULL,200);
CREATE TABLE t2(x,y,z);
INSERT INTO t2 VALUES(1,NULL,55);
SELECT c FROM t1 WHERE (a,b) IN (SELECT x,y FROM t2 WHERE z==55);
INSERT INTO t1 VALUES(2,3,400);
INSERT INTO t2 VALUES(2,3,55);
SELECT c FROM t1 WHERE (a,b) IN (SELECT x,y FROM t2 WHERE z==55);