使用复合索引代替单键索引，来避免单键有null值的情况_weixin_34130269的博客

SQL> select count(*) from t1;

  COUNT(*)
----------
   3229088

SQL> select count(*) from t1 where object_id is null;

  COUNT(*)
----------
        32

SQL> 
SQL> select count(*) from t1;