三大数据库 sequence 之华山论剑 (中篇)

2022 年 3 月 3 日
笔记
MySQL, Oracle, PostgreSQL

sequence 用法四 AUTO INCREMENT

通过 DEFAULT 还是需要手动创建 sequence。有没有更简单的用法呢？

当然，就是通过 AUTO INCREMENT 方式，自动创建 sequence，并且自动在 DEFAULT 中调用！

Oracle

同样，Oracle 也是 12c 开始支持 AUTO INCREMENT。

以下测试是在 Oracle Database 12c Release 12.2.0.1.0 中进行的。

SQL> CREATE TABLE tb_test4 (
    test_id NUMBER GENERATED BY DEFAULT ON NULL AS IDENTITY,
    test_order NUMBER
);  2    3    4  

Table created.

插入测试数据，可以看到预期的结果。

SQL> INSERT INTO tb_test4 (test_id,test_order) VALUES (DEFAULT,1);

1 row created.

SQL> INSERT INTO tb_test4 (test_order) VALUES (2);

1 row created.

SQL> COMMIT;

Commit complete.

SQL> SELECT * FROM tb_test4 ORDER BY 2 DESC;

   TEST_ID TEST_ORDER
---------- ------------
	 2	      2
	 1	      1

通过查询可以看到，系统自动创建了一个名为 ISEQ$$_254835（Oracle 自动生成的名字一般都不太友好）的 sequence，并将其设置为了 DEFAULT。

SQL> COL object_name FOR a30
COL object_type FOR a30SQL> 
SQL> SELECT object_name,object_type FROM user_objects;

OBJECT_NAME		       OBJECT_TYPE
------------------------------ ------------------------------
TB_TEST4		       TABLE
ISEQ$$_254835		       SEQUENCE
SQL> SET linesize 100
COL table_name FOR a30
COL column_name FOR a30
COL data_default FOR a30
SQL> SELECT table_name,column_name,data_default FROM user_tab_columns WHERE table_name = 'TB_TEST4';

TABLE_NAME		       COLUMN_NAME		      DATA_DEFAULT
------------------------------ ------------------------------ ------------------------------
TB_TEST4		       TEST_ID			      "TEST"."ISEQ$$_254857".nextval
TB_TEST4		       TEST_ORDER

通过以下数据字典可以看出，自动生成的 sequence 是与列对应的。

SQL> SET linesize 200
COL table_name FOR a30
COL column_name FOR a30
COL generation FOR a30
COL sequence_name FOR a30SQL> SQL> SQL> SQL> 
SQL> SELECT table_name,column_name,sequence_name FROM user_tab_identity_cols;

TABLE_NAME		       COLUMN_NAME		      SEQUENCE_NAME
------------------------------ ------------------------------ ------------------------------
TB_TEST4		       TEST_ID			      ISEQ$$_254835

自动生成的 sequence 是否可以删除呢？

通过如下实验，可以看到，Oracle 中系统自动生成的 sequence 不能单独删除。

SQL> DROP SEQUENCE ISEQ$$_254835;
DROP SEQUENCE ISEQ$$_254835
              *
ERROR at line 1:
ORA-32794: cannot drop a system-generated sequence

删除表会删除对应的 sequence 吗？会。表以 BIN 开头，代表 table 已被删除，但 PURGE RECYCLEBIN 后才会被彻底删除。

SQL> DROP TABLE tb_test4;

Table dropped.

SQL> SELECT table_name,column_name,sequence_name FROM user_tab_identity_cols;

TABLE_NAME		       COLUMN_NAME		      SEQUENCE_NAME
------------------------------ ------------------------------ ------------------------------
BIN$vXiAW/32gIHgU5KhXwowkg==$0 TEST_ID			      ISEQ$$_254835

SQL> SELECT object_name,object_type FROM USER_OBJECTS;

OBJECT_NAME		       OBJECT_TYPE
------------------------------ ------------------------------
ISEQ$$_254835		       SEQUENCE

SQL> PURGE RECYCLEBIN;

Recyclebin purged.

SQL> SELECT object_name,object_type FROM USER_OBJECTS;

no rows selected

SQL> SELECT table_name,column_name,sequence_name FROM user_tab_identity_cols;

no rows selected

下面测试表明，当在 INSERT 中指定列 test_id 为 NULL 时，会从 sequence 中取值。

这是 Oracle 中 GENERATED BY DEFAULT ON NULL 中的 ON NULL 决定的。

SQL> SELECT * FROM tb_test4 ORDER BY 2 DESC;

   TEST_ID TEST_ORDER
---------- ----------
	 6	    8

SQL> INSERT INTO tb_test4 (test_id,test_order) VALUES (NULL,9);

1 row created.

SQL> COMMIT;

Commit complete.

SQL> SELECT * FROM tb_test4 ORDER BY 2 DESC;

   TEST_ID TEST_ORDER
---------- ----------
	 7	    9
	 6	    8

以上示例中使用的是 GENERATED BY DEFAULT ON NULL，其他还有 GENERATED BY DEFAULT 和 GENERATED ALWAYS 。

GENERATED BY DEFAULT 与 GENERATED BY DEFAULT ON NULL 区别是，当主键插入 NULL 值时，GENERATED BY DEFAULT 会报错，如下：

SQL> CREATE TABLE tb_test5 (
    test_id NUMBER GENERATED BY DEFAULT AS IDENTITY,
    test_order NUMBER
);   2    3    4  

Table created.

SQL> INSERT INTO tb_test5 (test_id,test_order) VALUES (NULL,1);
INSERT INTO tb_test5 (test_id,test_order) VALUES (NULL,1)
                                                  *
ERROR at line 1:
ORA-01400: cannot insert NULL into ("TEST"."TB_TEST5"."TEST_ID")

GENERATED ALWAYS 是插入时，

若主键指定值会报错：

SQL> CREATE TABLE tb_test6 (
    test_id NUMBER GENERATED ALWAYS AS IDENTITY,
    test_order NUMBER
);   2    3    4  

Table created.

SQL> INSERT INTO tb_test6 (test_id,test_order) VALUES (1,1);
INSERT INTO tb_test6 (test_id,test_order) VALUES (1,1)
                      *
ERROR at line 1:
ORA-32795: cannot insert into a generated always identity column

SQL> INSERT INTO tb_test6 (test_id,test_order) VALUES (NULL,2);
INSERT INTO tb_test6 (test_id,test_order) VALUES (NULL,2)
                      *
ERROR at line 1:
ORA-32795: cannot insert into a generated always identity column

但指定 DEFAULT 可以：

SQL> INSERT INTO tb_test6 (test_id,test_order) VALUES (DEFAULT,3);

1 row created.
SQL> COMMIT;

Commit complete.

SQL> SELECT * FROM tb_test6 ORDER BY 2 DESC;

   TEST_ID TEST_ORDER
---------- ----------
	 1	    3

PostgreSQL

在 PostgreSQL 中同样可以，甚至更简单，SERIAL 一个单词就够了。

创建表

alvindb=> CREATE TABLE tb_test4 (
alvindb(>     test_id SERIAL PRIMARY KEY,
alvindb(>     test_order INTEGER
alvindb(> );
CREATE TABLE

插入测试数据，结果符合预期。

alvindb=> INSERT INTO tb_test4 (test_id,test_order) VALUES (DEFAULT,1);
INSERT 0 1
alvindb=> INSERT INTO tb_test4 (test_order) VALUES (2);
INSERT 0 1
alvindb=> SELECT * FROM tb_test4 ORDER BY 2 DESC;
 test_id | test_order 
---------+--------------
       2 |            2
       1 |            1
(2 rows)

通过如下方式查看表结构，可以看到，加了 SERIAL 以后，自动创建了 sequence tb_test4_test_id_seq（PostgreSQL 自动生成的名字一般比较友好），并将其设置为了 DEFAULT。

alvindb=> \d+ tb_test4
                                                    Table "public.tb_test4"
   Column   |  Type   | Collation | Nullable |                  Default                  | Storage | Stats target | Description 
------------+---------+-----------+----------+-------------------------------------------+---------+--------------+-------------
 test_id    | integer |           | not null | nextval('tb_test4_test_id_seq'::regclass) | plain   |              | 
 test_order | integer |           |          |                                           | plain   |              | 
Indexes:
    "tb_test4_pkey" PRIMARY KEY, btree (test_id)

查看 sequence 定义看的 ‘Owned by’，可以看到，自动生成的 sequence 与对应列也是绑定的。

即 ‘Owned by’ 也代表删除表或列的时候，对应的 sequence 也会被删除。

alvindb=> \d+ tb_test4_test_id_seq
                Sequence "public.tb_test4_test_id_seq"
  Type   | Start | Minimum |  Maximum   | Increment | Cycles? | Cache 
---------+-------+---------+------------+-----------+---------+-------
 integer |     1 |       1 | 2147483647 |         1 | no      |     1
Owned by: public.tb_test4.test_id

下面测试删除表后，自动生成的 sequence 也会被删除。

alvindb=> DROP TABLE tb_test4;
DROP TABLE
alvindb=> \d+ tb_test4_test_id_seq
Did not find any relation named "tb_test4_test_id_seq".
alvindb=>

从下面的例子中可以看出，删除 sequence 效果与用法三中一样，表列的 DEFAULT 也被删除了。

alvindb=> CREATE TABLE tb_test4 (
alvindb(>     test_id SERIAL PRIMARY KEY,
alvindb(>     test_order INTEGER
alvindb(> );
CREATE TABLE
alvindb=> DROP SEQUENCE tb_test4_test_id_seq;
ERROR:  cannot drop sequence tb_test4_test_id_seq because other objects depend on it
DETAIL:  default value for column test_id of table tb_test4 depends on sequence tb_test4_test_id_seq
HINT:  Use DROP ... CASCADE to drop the dependent objects too.
alvindb=> DROP SEQUENCE tb_test4_test_id_seq CASCADE;
NOTICE:  drop cascades to default value for column test_id of table tb_test4
DROP SEQUENCE
alvindb=> \d+ tb_test4
                                                    Table "public.tb_test4"
   Column   |  Type   | Collation | Nullable |                  Default                  | Storage | Stats target | Description 
------------+---------+-----------+----------+-------------------------------------------+---------+--------------+-------------
 test_id    | integer |           | not null | nextval('tb_test4_test_id_seq'::regclass) | plain   |              | 
 test_order | integer |           |          |                                           | plain   |              | 
Indexes:
    "tb_test4_pkey" PRIMARY KEY, btree (test_id)

与上述用法三中手动创建 sequence 不同的是，使用 SERIAL 的话：

a. 自动生成的 sequence 与对应列是绑定的

b. 删除表后，自动生成的 sequence 也会被删除，避免无用的 sequence 的存在

根据 PostgreSQL官方文档，

CREATE TABLE tablename (
    colname SERIAL
);

与如下三个 SQL 等价：

CREATE SEQUENCE tablename_colname_seq AS integer;
CREATE TABLE tablename (
    colname integer NOT NULL DEFAULT nextval('tablename_colname_seq')
);
ALTER SEQUENCE tablename_colname_seq OWNED BY tablename.colname;

下面测试表明，当在 INSERT 中指定列 test_id 为 NULL 时，会报错。

从上面 SERIAL 等价的 SQL 中可以看出，PostgreSQL 只是将上述三个 SQL 精简为 SERIAL 关键字，与 Oracle 中 AUTO INCREMENT 本质是不同的。

alvindb=> INSERT INTO tb_test4 (test_id,test_order) VALUES (NULL,9);
ERROR:  null value in column "test_id" violates not-null constraint
DETAIL:  Failing row contains (null, 9).
alvindb=> INSERT INTO tb_test3 (test_id,test_order) VALUES (NULL,9);
ERROR:  null value in column "test_id" violates not-null constraint
DETAIL:  Failing row contains (null, 9).

那么 PostgreSQL 是哪个版本开始支持 SERIAL 的呢？

PostgreSQL 官网文档中列出的最早的版本是 PostgreSQL 8.0，在这个文档中，已支持这种用法。

这是 PostgreSQL 8.0 文档中的例子

CREATE TABLE cinemas (
        id serial,
        name text,
        location text
) TABLESPACE diskvol1;

根据 PostgreSQL 官网， PostgreSQL Release 8.0 发布时间是 2005-01-19。

根据 Wikipedia， Oracle Database 12c Release 1 是 July 2014 发布的。

即 PostgreSQL 2005 年已支持 sequence 的 AUTO INCREMENT 用法，九年后，Oracle 也支持了。

MySQL

MySQL 用 AUTO_INCREMENT 关键字。

如下示例。

mysql> CREATE TABLE tb_test4 (
    ->     test_id INTEGER NOT NULL AUTO_INCREMENT PRIMARY KEY,
    ->     test_order INTEGER
    -> );
Query OK, 0 rows affected (0.01 sec)
mysql> INSERT INTO tb_test4 (test_id,test_order) VALUES (DEFAULT,1);
Query OK, 1 row affected (0.00 sec)

mysql> INSERT INTO tb_test4 (test_order) VALUES (2);
Query OK, 1 row affected (0.01 sec)

mysql> SELECT * FROM tb_test4 ORDER BY 2 DESC;
+---------+--------------+
| test_id | test_order |
+---------+--------------+
|       2 |            2 |
|       1 |            1 |
+---------+--------------+
2 rows in set (0.00 sec)

从 SHOW CREATE TABLE 中可以看出，与 Oracle 和 PostgresSQL 不同的是，MySQL 并未创建单独的 sequence。

mysql> SHOW CREATE TABLE tb_test4;
+----------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| Table    | Create Table                                                                                                                                                                                    |
+----------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
| tb_test4 | CREATE TABLE `tb_test4` (
  `test_id` int(11) NOT NULL AUTO_INCREMENT,
  `test_order` int(11) DEFAULT NULL,
  PRIMARY KEY (`test_id`)
) ENGINE=InnoDB AUTO_INCREMENT=102 DEFAULT CHARSET=latin1 |
+----------+-------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------+
1 row in set (0.00 sec)

同 Oracle 中一样，当在 INSERT 中指定列 test_id 为 NULL 时，会正常自增。这是 MySQL AUTO INCREMENT 决定的。

mysql> INSERT INTO tb_test4 (test_id,test_order) VALUES (NULL,9);
Query OK, 1 row affected (0.01 sec)

mysql> SELECT * FROM tb_test5 ORDER BY 2 DESC;
+---------+------------+
| test_id | test_order |
+---------+------------+
|       1 |          8 |
+---------+------------+
1 row in set (0.00 sec)

下面通过以下测试用例，比较一下 INSERT/UPDATE/DELETE/TRUNCATE 对 sequence 的影响。

INSERT INTO tb_test4 (test_order) VALUES (1);
INSERT INTO tb_test4 (test_id,test_order) VALUES (100,2);
INSERT INTO tb_test4 (test_order) VALUES (3);
UPDATE tb_test4 SET test_id = 200 WHERE test_order = 3;
INSERT INTO tb_test4 (test_order) VALUES (5);
INSERT INTO tb_test4 (test_order) VALUES (6);
SELECT * FROM tb_test4 ORDER BY 2 DESC;
DELETE FROM tb_test4 WHERE test_order = 5;
DELETE FROM tb_test4 WHERE test_order = 6;
INSERT INTO tb_test4 (test_order) VALUES (7);
TRUNCATE TABLE tb_test4;
INSERT INTO tb_test4 (test_order) VALUES (8);

通过下面详细的测试，得出如下结论：

Oracle 和 PostgreSQL 测试结果相同，INSERT 比当前 sequence 大的值，还有 UPDATE/DELETE/TRUNCATE，均对其 sequence 无影响。

而在MySQL 5.7 InnoDB ENGINE 中，INSERT 比当前 sequence 大的值和 TRUNCATE 对其 sequence 有影响，而 UPDATE/DELETE 对其 sequence 无影响。

而在MySQL 5.7 MYISAM ENGINE 中，INSERT 比当前 sequence 大的值和 UPDATE/TRUNCATE 对其 sequence 有影响，而 DELETE 对其 sequence 无影响。

MySQL 5.7 InnoDB ENGINE

以下是 MySQL 5.7 InnoDB ENGINE 中的运行结果

mysql> INSERT INTO tb_test4 (test_order) VALUES (1);
Query OK, 1 row affected (0.00 sec)

mysql> SELECT * FROM tb_test4 ORDER BY 2 DESC;
+---------+------------+
| test_id | test_order |
+---------+------------+
|       1 |          1 |
+---------+------------+
1 row in set (0.00 sec)

mysql> SELECT last_insert_id();
+------------------+
| LAST_INSERT_ID() |
+------------------+
|                1 |
+------------------+
1 row in set (0.00 sec)

mysql> INSERT INTO tb_test4 (test_id,test_order) VALUES (100,2);
Query OK, 1 row affected (0.00 sec)

mysql> SELECT last_insert_id();
+------------------+
| last_insert_id() |
+------------------+
|                1 |
+------------------+
1 row in set (0.00 sec)

mysql> INSERT INTO tb_test4 (test_order) VALUES (3);
Query OK, 1 row affected (0.00 sec)

mysql> SELECT last_insert_id();
+------------------+
| last_insert_id() |
+------------------+
|              101 |
+------------------+
1 row in set (0.00 sec)

mysql> SELECT * FROM tb_test4 ORDER BY 2 DESC;
+---------+------------+
| test_id | test_order |
+---------+------------+
|     101 |          3 |
|     100 |          2 |
|       1 |          1 |
+---------+------------+
3 rows in set (0.00 sec)
mysql> UPDATE tb_test4 SET test_id = 200 WHERE test_order = 3;
Query OK, 1 row affected (0.00 sec)
Rows matched: 1  Changed: 1  Warnings: 0

mysql> SELECT last_insert_id();
+------------------+
| last_insert_id() |
+------------------+
|              101 |
+------------------+
1 row in set (0.00 sec)

mysql> INSERT INTO tb_test4 (test_order) VALUES (5);
Query OK, 1 row affected (0.00 sec)

mysql> SELECT last_insert_id();
+------------------+
| last_insert_id() |
+------------------+
|              102 |
+------------------+
1 row in set (0.00 sec)

mysql> SELECT * FROM tb_test4 ORDER BY 2 DESC;
+---------+------------+
| test_id | test_order |
+---------+------------+
|     102 |          5 |
|     200 |          3 |
|     100 |          2 |
|       1 |          1 |
+---------+------------+
4 rows in set (0.00 sec)
mysql> INSERT INTO tb_test4 (test_order) VALUES (6);
Query OK, 1 row affected (0.01 sec)

mysql> SELECT * FROM tb_test4 ORDER BY 2 DESC;
+---------+------------+
| test_id | test_order |
+---------+------------+
|     103 |          6 |
|     102 |          5 |
|     200 |          3 |
|     100 |          2 |
|       1 |          1 |
+---------+------------+
5 rows in set (0.00 sec)

mysql> SELECT last_insert_id();
+------------------+
| last_insert_id() |
+------------------+
|              103 |
+------------------+
1 row in set (0.00 sec)

mysql> DELETE FROM tb_test4 WHERE test_order = 5;
Query OK, 1 row affected (0.01 sec)

mysql> SELECT last_insert_id();
+------------------+
| last_insert_id() |
+------------------+
|              103 |
+------------------+
1 row in set (0.00 sec)

mysql> DELETE FROM tb_test4 WHERE test_order = 6;
Query OK, 1 row affected (0.00 sec)

mysql> SELECT last_insert_id();
+------------------+
| last_insert_id() |
+------------------+
|              103 |
+------------------+
1 row in set (0.00 sec)

mysql> SELECT * FROM tb_test4 ORDER BY 2 DESC;
+---------+------------+
| test_id | test_order |
+---------+------------+
|     200 |          3 |
|     100 |          2 |
|       1 |          1 |
+---------+------------+
3 rows in set (0.00 sec)

mysql> INSERT INTO tb_test4 (test_order) VALUES (7);
Query OK, 1 row affected (0.01 sec)

mysql> SELECT last_insert_id();
+------------------+
| last_insert_id() |
+------------------+
|              104 |
+------------------+
1 row in set (0.00 sec)

mysql> SELECT * FROM tb_test4 ORDER BY 2 DESC;
+---------+------------+
| test_id | test_order |
+---------+------------+
|     104 |          7 |
|     200 |          3 |
|     100 |          2 |
|       1 |          1 |
+---------+------------+
4 rows in set (0.00 sec)
mysql> TRUNCATE TABLE tb_test4;
Query OK, 0 rows affected (0.00 sec)

mysql> SELECT last_insert_id();
+------------------+
| last_insert_id() |
+------------------+
|              104 |
+------------------+
1 row in set (0.00 sec)

mysql> INSERT INTO tb_test4 (test_order) VALUES (8);
Query OK, 1 row affected (0.00 sec)

mysql> SELECT last_insert_id();
+------------------+
| last_insert_id() |
+------------------+
|                1 |
+------------------+
1 row in set (0.00 sec)

mysql> SELECT * FROM tb_test4 ORDER BY 2 DESC;
+---------+------------+
| test_id | test_order |
+---------+------------+
|       1 |          8 |
+---------+------------+
1 row in set (0.00 sec)