text: -- original: coveridxscan.test
  -- credit:   http://www.sqlite.org/src/tree?ci=trunk&name=test
  
  CREATE TABLE t1(a,b,c);
      INSERT INTO t1 VALUES(5,4,3), (4,8,2), (3,2,1);
      CREATE INDEX t1ab ON t1(a,b);
      CREATE INDEX t1b ON t1(b);
      SELECT a FROM t1
  ;SELECT a, c FROM t1
  ;SELECT b FROM t1
  ;SELECT a FROM t1
  ;SELECT a, c FROM t1
  ;SELECT b FROM t1
  ;SELECT a FROM t1
  ;SELECT a, c FROM t1
  ;SELECT b FROM t1
  ;SELECT a FROM t1
  ;SELECT a, c FROM t1
  ;SELECT b FROM t1;