Oracle 和 Mysql 的索引在Null字段上处理的异同

ORACLE：

SQL> create table tab2(c1 number, c2 number, c3 varchar2(10));

表が作成されました。

SQL> declare
  a number;
begin
  a := 1;
  for i in 1 .. 500 loop
    for j in 1 .. 1000 loop
      insert into tab2 values(a,j,'a');
      commit;
      a := a+1;
    end loop;
  end loop;
end;
/

PL/SQLプロシージャが正常に完了しました。

SQL> create index ind2_2 on tab2(c2);

索引が作成されました。

SQL> insert into tab2 values(9999,null,'test');

1行が作成されました。

SQL> commit;

コミットが完了しました。

SQL> exec dbms_stats.gather_table_stats(ownname=>'TEST',tabname=>'TAB2',cascade=>TRUE);

PL/SQLプロシージャが正常に完了しました。

SQL> set lin 150 pages 9999
SQL> set autot traceonly exp
SQL> select count(*) from tab2 where c2 is null;

実行計画
----------------------------------------------------------
Plan hash value: 2781695375

---------------------------------------------------------------------------
| Id  | Operation          | Name | Rows  | Bytes | Cost (%CPU)| Time     |
---------------------------------------------------------------------------
|   0 | SELECT STATEMENT   |      |     1 |     4 |   310   (1)| 00:00:01 |
|   1 |  SORT AGGREGATE    |      |     1 |     4 |            |          |
|*  2 |   TABLE ACCESS FULL| TAB2 |     1 |     4 |   310   (1)| 00:00:01 |
---------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   2 - filter("C2" IS NULL)

SQL> select count(*) from tab2 where c2=10;

実行計画
----------------------------------------------------------
Plan hash value: 3563712581

----------------------------------------------------------------------------
| Id  | Operation         | Name   | Rows  | Bytes | Cost (%CPU)| Time     |
----------------------------------------------------------------------------
|   0 | SELECT STATEMENT  |        |     1 |     4 |     4   (0)| 00:00:01 |
|   1 |  SORT AGGREGATE   |        |     1 |     4 |            |          |
|*  2 |   INDEX RANGE SCAN| IND2_2 |   500 |  2000 |     4   (0)| 00:00:01 |
----------------------------------------------------------------------------

Predicate Information (identified by operation id):
---------------------------------------------------

   2 - access("C2"=10)
   

Mysql（Innodb）：

mysql> create table tab2(c1 int, c2 int, c3 varchar(10));
Query OK, 0 rows affected (0.01 sec)

mysql> delimiter //
mysql> create procedure my_procedure()
-> begin
-> DECLARE n int DEFAULT 1;
-> WHILE n < 1001 DO
-> insert into tab2(c1,c2,c3) value (n,n,'desc');
-> set n = n + 1;
-> END WHILE;
-> end
-> //
Query OK, 0 rows affected (0.01 sec)

mysql> delimiter ;
mysql> call my_procedure;
Query OK, 1 row affected (0.84 sec)

mysql> create index ind2_2 on tab2(c2);
Query OK, 0 rows affected (0.02 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> insert into tab2 values(9999,null,'test');
Query OK, 1 row affected (0.00 sec)

mysql> explain select count(*) from tab2 where c2=10;
+----+-------------+-------+------------+------+---------------+--------+---------+-------+------+----------+-------------+
| id | select_type | table | partitions | type | possible_keys | key    | key_len | ref   | rows | filtered | Extra       |
+----+-------------+-------+------------+------+---------------+--------+---------+-------+------+----------+-------------+
|  1 | SIMPLE      | tab2  | NULL       | ref  | ind2_2        | ind2_2 | 5       | const |    1 |   100.00 | Using index |
+----+-------------+-------+------------+------+---------------+--------+---------+-------+------+----------+-------------+
1 row in set, 1 warning (0.00 sec)

mysql> explain select count(*) from tab2 where c2 is null;
+----+-------------+-------+------------+------+---------------+--------+---------+-------+------+----------+--------------------------+
| id | select_type | table | partitions | type | possible_keys | key    | key_len | ref   | rows | filtered | Extra                    |
+----+-------------+-------+------------+------+---------------+--------+---------+-------+------+----------+--------------------------+
|  1 | SIMPLE      | tab2  | NULL       | ref  | ind2_2        | ind2_2 | 5       | const |    1 |   100.00 | Using where; Using index |
+----+-------------+-------+------------+------+---------------+--------+---------+-------+------+----------+--------------------------+
1 row in set, 1 warning (0.00 sec)

结论：

Oracle的B-tree索引不存储Null，所以“c2 is null”条件的检索不能从索引中受益。
Mysql的B+tree索引也不直接不存储Null，但是“c2 is null”条件的检索能从索引中受益。

https://dev.mysql.com/doc/ref...

ref_or_null works by first doing a read on the reference key, and then a separate search for rows with a NULL key value.