Let’s talk about how to modify Oracle sequences

Oracle is a very popular relational database management system. In Oracle, a sequence is a very useful object that can be used to generate a series of unique integer values. Normally, sequences are used in Oracle to set primary key values ​​or other columns that require unique values. But sometimes it is necessary to modify the sequence, such as adjusting the step size, starting value, etc. of the sequence. This article will introduce how to modify Oracle sequences.

Basic syntax of Oracle sequence

To create a sequence in Oracle, you can use the following syntax:

CREATE SEQUENCE sequence_name
  [START WITH n]
  [INCREMENT BY n]
  [MAXVALUE n | NOMAXVALUE]
  [MINVALUE n | NOMINVALUE]
  [CYCLE | NOCYCLE]
  [CACHE n | NOCACHE];

The following is the meaning of each parameter:

• sequence_name: The name of the sequence to create.
• START WITH: Specify the starting value of the sequence, the default value is 1.
• INCREMENT BY: The amount incremented each time the sequence is called to generate the next value. The default value is 1.
• MAXVALUE: The maximum value of the sequence generator, the default value is 10^28-1, it can be set to no upper limit through the NOMAXVALUE keyword.
• NOMAXVALUE: The sequence generator has no upper limit.
• MINVALUE: The minimum value of the sequence generator, the default value is 1, it can be set to have no lower limit using the NOMINVALUE keyword.
• NOMINVALUE: The sequence generator has no lower limit.
• CYCLE: When the sequence reaches the maximum value, the sequence value is generated again from the starting value. The default is NOCYCLE.
• NOCYCLE: Stop generating sequence values ​​when the sequence reaches its maximum value.
• CACHE: Specifies the number of pre-allocated sequence values. By default, the sequence generator directly accesses the database to generate sequence values, but when frequent access to generate sequence values ​​is required, CACHE can be used for optimization.

Methods to modify the Oracle sequence

Modifying the Oracle sequence can be achieved by the following two methods:

• Modify the sequence attributes through the ALTER SEQUENCE statement
• Delete and re-create the sequence through the DROP/CREATE statement

The two methods are introduced below.

Method 1: Modify sequence attributes through ALTER SEQUENCE statement

In Oracle, you can use the ALTER SEQUENCE statement to modify the attributes of an existing sequence. For example, the following statement can modify the starting value of the sequence MY_SEQUENCE from 1 to 101:

ALTER SEQUENCE MY_SEQUENCE START WITH 101;

Other available sequence attributes include INCREMENT BY, MAXVALUE, MINVALUE, CYCLE, etc. For example, the following statement can modify the step size of the sequence from 1 to 10:

ALTER SEQUENCE MY_SEQUENCE INCREMENT BY 10;

If you need to prohibit the loop from generating sequence values, you can use the following statement:

ALTER SEQUENCE MY_SEQUENCE NOCYCLE;

In this way, when the sequence reaches the maximum value, no new sequence values ​​will be generated.

Method 2: Delete and re-create the sequence through the DROP/CREATE statement

Another way to modify a sequence is to delete the existing sequence and re-create a sequence with new attributes. To avoid dropping any tables or views that are currently using a sequence, you must disable or drop them before modifying the sequence.

The following is a sample code for this method:

-- 禁用序列
ALTER TABLE my_table DISABLE CONSTRAINT my_table_id_pk;

-- 删除序列
DROP SEQUENCE my_sequence;

-- 创建新序列
CREATE SEQUENCE my_sequence
    INCREMENT BY 10
    START WITH 101
    MAXVALUE 1000
    NOCYCLE
    CACHE 20;

-- 启用序列
ALTER TABLE my_table ENABLE CONSTRAINT my_table_id_pk;

In this example, we first disable the primary key of the table using the sequence; then delete the existing sequence; create a new sequence , and set the new value; finally enable the table's primary key constraints.

It is important to note that when an existing sequence is deleted and recreated, the sequence name and all parameters must be the same as the original sequence (except for the parameters that need to be changed) to avoid any problems in other code and applications .

Conclusion

This article introduces the basic concepts and modification methods of Oracle sequences. By using the ALTER SEQUENCE statement or the DROP/CREATE statement, you can easily modify the existing sequence and set new values ​​as needed. When changing sequences, you need to be careful and make sure you follow best practices to avoid unnecessary problems.

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