ANBOB™

最近几年数据库市场百花齐放，在做跨数据库迁移的数据库选型时，除了性能、稳定、安全、运维、功能、可扩展外，像开发中对于值的处理往往容易被人忽视， 之前写过一篇关于PG区别Oracle在SQL解析缓存的笔记《PostgreSQL 12 : Prepare statement和plan_cache_mode 参数》，这里记录一下null 值在这几个数据库中的区别。

软件版本：Oracle 21c 、SQL Server 2019 、MySQL 8.0 、Mariadb 10.6 、PostGreSQL 13、OpenGauss 2.0

创建测试用例表

CREATE TABLE tab_null(id int, name char(10));
 
INSERT INTO tab_null VALUES(1,'anbob');
INSERT INTO tab_null VALUES(2,NULL);

测试 数据过滤

# oracle
SQL> select * from tab_null where name is null;
        ID NAME
---------- ----------
         2

SQL> select * from tab_null where name is not null;
        ID NAME
---------- ----------
         1 anbob

SQL> select * from tab_null where name=null;
no rows selected

SQL> select * from tab_null where null=null;
no rows selected

# postgresql
postgres=# select * from tab_null where name is null;
 id | name
----+------
  2 |
(1 row)

postgres=# select * from tab_null where name is not null;
 id |    name
----+------------
  1 | anbob
(1 row)

postgres=# select * from tab_null where name=null;
 id | name
----+------
(0 rows)

postgres=# select * from tab_null where null=null;
 id | name
----+------
(0 rows)

# MySQL/MariaDB
mysql> select * from tab_null where name is null;
+------+------+
| id   | name |
+------+------+
|    2 | NULL |
+------+------+
1 row in set (0.00 sec)

mysql> select * from tab_null where name is not null;
+------+-------+
| id   | name  |
+------+-------+
|    1 | anbob |
+------+-------+
1 row in set (0.00 sec)

mysql> select * from tab_null where name=null;
Empty set (0.00 sec)

mysql> select * from tab_null where null=null;
Empty set (0.00 sec)

# SQL Server

select * from tab_null where name is null;
        ID NAME
---------- ----------
         2

select * from tab_null where name is not null;
        ID NAME
---------- ----------
         1 anbob

select * from tab_null where name=null;
no rows selected

select * from tab_null where null=null;
no rows selected

Note:
可见所有数据库的结果是一样的。

另一种另类

# Mysql
mysql> select 1 from tab_null where 1 not in (null);
Empty set (0.00 sec)

mysql> select 1 from tab_null where null not in (null);
Empty set (0.00 sec)

mysql> select 1 from tab_null where null  in (null);
Empty set (0.00 sec)

mysql> select 1 from tab_null where exists(select null from dual);
+---+
| 1 |
+---+
| 1 |
| 1 |
| 1 |
+---+
3 rows in set (0.05 sec)

Note:
这类在4个库返回也是一样的，上面只附了MySQL,不再展示其它库。

唯一约束

# oracle
SQL> alter table tab_null add constraint c_tab_null_name_uni  unique(name);
Table altered.

SQL> INSERT INTO tab_null VALUES(3,NULL);
1 row created.

SQL> select * from tab_null;
        ID NAME
---------- ----------
         1 anbob
         2
         3
# postgresql
postgres=# alter table tab_null add constraint c_tab_null_name_uni  unique(name);
ALTER TABLE
postgres=# INSERT INTO tab_null VALUES(3,NULL);
INSERT 0 1
postgres=# select * from tab_null;
 id |    name
----+------------
  1 | anbob
  2 |
  3 |
(3 rows)

# MySQL/MariaDB
mysql> alter table tab_null add constraint c_tab_null_name_uni  unique(name);
Query OK, 0 rows affected (0.14 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> INSERT INTO tab_null VALUES(3,NULL);
Query OK, 1 row affected (0.01 sec)

mysql> select * from tab_null;
+------+-------+
| id   | name  |
+------+-------+
|    1 | anbob |
|    2 | NULL  |
|    3 | NULL  |
+------+-------+
3 rows in set (0.00 sec)

# SQL SERVER
alter table tab_null add constraint c_tab_null_name_uni  unique(name);

INSERT INTO tab_null VALUES(3,NULL);
Msg 2627 Level 14 State 1 Line 12
Violation of UNIQUE KEY constraint 'c_tab_null_name_uni'. 
Cannot insert duplicate key in object 'dbo.tab_null'. The duplicate key value is (<NULL>).

Note:
这里只有SQL SERVER提示一个表中的null和null是重复记录， 其它库可以正常insert 多个 null 到有唯一约束的表。

NULL使用索引

# MySQL/MariaDB
mysql> alter table tab_null drop constraint c_tab_null_name_uni;
Query OK, 0 rows affected (0.05 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> create index idx_tab_null_name on tab_null(name);
Query OK, 0 rows affected (0.05 sec)
Records: 0  Duplicates: 0  Warnings: 0

mysql> explain select * from tab_null where name is null;
+----+-------------+----------+------------+------+-------------------+-------------------+---------+-------+------+----------+-----------------------+
| id | select_type | table    | partitions | type | possible_keys     | key               | key_len | ref   | rows | filtered | Extra                 |
+----+-------------+----------+------------+------+-------------------+-------------------+---------+-------+------+----------+-----------------------+
|  1 | SIMPLE      | tab_null | NULL       | ref  | idx_tab_null_name | idx_tab_null_name | 41      | const |    2 |   100.00 | Using index condition |
+----+-------------+----------+------------+------+-------------------+-------------------+---------+-------+------+----------+-----------------------+
1 row in set, 1 warning (0.00 sec)

# PostgreSQL
postgres=# alter table tab_null drop constraint c_tab_null_name_uni;
ALTER TABLE
postgres=# create index idx_tab_null_name on tab_null(name);
CREATE INDEX
postgres=# explain analyze select * from tab_null where name is null;
                                            QUERY PLAN
---------------------------------------------------------------------------------------------------
 Seq Scan on tab_null  (cost=0.00..1.03 rows=1 width=18) (actual time=0.008..0.009 rows=2 loops=1)
   Filter: (name IS NULL)
   Rows Removed by Filter: 1
 Planning Time: 0.130 ms
 Execution Time: 0.019 ms
-- 因为小表代价，这是使用了seq scan 全表扫，下面往表里insert一些数据

postgres=# insert into tab_null select generate_series,generate_series||'anbob' from generate_series(101,10000);
INSERT 0 9900
postgres=# analyze tab_null;
ANALYZE
postgres=# explain analyze select * from tab_null where name is null;
                                                         QUERY PLAN
-----------------------------------------------------------------------------------------------------------------------------
 Index Scan using idx_tab_null_name on tab_null  (cost=0.29..9.64 rows=2 width=15) (actual time=0.010..0.011 rows=2 loops=1)
   Index Cond: (name IS NULL)
 Planning Time: 0.199 ms
 Execution Time: 0.031 ms
(4 rows)

# Oracle
SQL>alter table tab_null drop constraint c_tab_null_name_uni;
Table altered.

SQL> create index idx_tab_null_name on tab_null(name);
Index created.

SQL> explain plan for select /*+index(t)*/ * from tab_null t where name is null;

Explained.

SQL> @x2

PLAN_TABLE_OUTPUT
------------------------------------------------------------------------------------------------
Plan hash value: 2647411751

------------------------------------------------------------------------------
| Id  | Operation         | Name     | Rows  | Bytes | Cost (%CPU)| Time     |
------------------------------------------------------------------------------
|   0 | SELECT STATEMENT  |          |     2 |    50 |     2   (0)| 00:00:01 |
|*  1 |  TABLE ACCESS FULL| TAB_NULL |     2 |    50 |     2   (0)| 00:00:01 |
------------------------------------------------------------------------------

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

   1 - filter("NAME" IS NULL)

Hint Report (identified by operation id / Query Block Name / Object Alias):
Total hints for statement: 1 (U - Unused (1))
---------------------------------------------------------------------------

   1 -  SEL$1 / "T"@"SEL$1"
         U -  index(t)

解决这个问题可以增加常数的复合索引

SQL> create index idx_tab_null_name_0 on tab_null(name,0);
Index created.

SQL> explain plan for select /*+index(t)*/ * from tab_null t where name is null;
Explained.

SQL> @x2
PLAN_TABLE_OUTPUT
-----------------------------------------------------------------------------------------------------------
Plan hash value: 2247804559

-----------------------------------------------------------------------------------------------------------
| Id  | Operation                           | Name                | Rows  | Bytes | Cost (%CPU)| Time     |
-----------------------------------------------------------------------------------------------------------
|   0 | SELECT STATEMENT                    |                     |     2 |    50 |     2   (0)| 00:00:01 |
|   1 |  TABLE ACCESS BY INDEX ROWID BATCHED| TAB_NULL            |     2 |    50 |     2   (0)| 00:00:01 |
|*  2 |   INDEX RANGE SCAN                  | IDX_TAB_NULL_NAME_0 |     1 |       |     1   (0)| 00:00:01 |
-----------------------------------------------------------------------------------------------------------
Predicate Information (identified by operation id):
---------------------------------------------------
   2 - access("NAME" IS NULL)

# SQL SERVER
SET statistics profile on;
SELECT * FROM tab_null with (index(idx_tab_null_name)) Where name IS NULL;

Note:
对于is null的谓词条件注意MySQL、MariaDB即使很少的记录也使用的index感觉更像是RULE CBO；PostgreSQL开始使用了全表扫，也可能是基于代价的估算，全表扫要优于使用索引，因为pg默认没有像oracle,sql server, mysql 自带的hint可以强制使用索引，后来我们填充了更多的数据，PostgreSQL也使用上了索引，当然PG 也有扩展pg_hint_plan可以实现，不过PG认为用户中使用hint干扰优化器是不好的习惯，不应该那么做；Oracle数据库因为单列索引不会储存null值，所以is null 即使使用hint 也无法使用索引，需要优化小技巧，增加常量的复合索引使用索引； SQL Server同样我们在加hint with (index(idx_tab_null_name))后，执行计划中的Index Seek 也可以确认用上了索引。

字符串拼接

# sql server 2019
select null+'anbob'

-------------
null

# mariadb 10.6
select null+'anbob'

-------------
null

# mysql 8.0
select null+'anbob'

-------------
null

# postgres 13
postgres=# select 'anbob'||null;
 ?column?
----------

(1 row)

# oracle 21c
SQL> select null||'anbob' from dual;

NULL|
-----
anbob

# OpenGauss
[og@oel7db1 data]$ gsql -d anbob -p 15432
gsql ((openGauss 2.0.0 build 78689da9) compiled at 2021-03-31 21:04:03 commit 0 last mr  )
NOTICE : The password has been expired, please change the password.
Non-SSL connection (SSL connection is recommended when requiring high-security)
Type "help" for help.
                                  ^
anbob=# select null||'anbob' ;
 ?column?
----------
 anbob
(1 row)

Note:
注意也是只有Oracle不同于其它库，在null值和其它字符串拼接后可以正常返回部分有效值，而其它所有库全部返回空。

总结：

对于null值的谓词过滤条件时（IS NULL、IS NOT NULL， >,< ,=），4个数据库返回数据记录是一样的;

对于null 值的唯一约束，只有SQL Server不允许null 记录重复，而其它数据库不限制；

对于is null使用Btree索引， 只有Oracle是默认无法索引的（需要创建常量的复合索引）,其它数据库优化器在认为合适时可以正常使用索引；

对于null与字符串拼接就更有意思，sql server\postgresql\mysql在与null拼接后返回null, Oracle返回是字符串的值，而且基于Postgresql的OpenGauss返回和Oracle相同也是字符串，看来是与oracle做过兼容性修改。

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