关于mysql: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プロシージャが失常に完了しました。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プロシージャが失常に完了しました。SQL> set lin 150 pages 9999SQL> set autot traceonly expSQL> 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: 0mysql> 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)

论断：

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害。
Mysql的B+tree索引也不间接不存储Null，然而“c2 is null”条件的检索能从索引中受害。

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.