NEXTVAL

 In Oracle, NEXTVAL is a sequence function used to generate the next value in a sequence, which is typically used for automatically generating unique numbers for primary keys or other purposes where unique identifiers are needed. Sequences are database objects that are used to produce a series of numbers. They can be incremented in different ways (e.g., by a certain step size, or even cyclically).

Here’s a detailed breakdown of the NEXTVAL function in Oracle:

1. What is a Sequence?

A sequence is an object in Oracle that generates a sequence of numeric values in an ordered fashion. It's commonly used for generating unique values for primary keys in tables. A sequence can be defined with several options, such as:

• START WITH: The initial value of the sequence.
• INCREMENT BY: The amount by which the sequence value is increased.
• CACHE: Specifies how many sequence numbers Oracle should preallocate and store in memory for faster access.
• CYCLE: If this option is enabled, the sequence will restart from the START WITH value once it reaches the maximum value.
• MAXVALUE / MINVALUE: The range of values for the sequence.

2. Using NEXTVAL

The NEXTVAL function is used to get the next value from a sequence.

Syntax:

sequence_name.NEXTVAL

• sequence_name is the name of the sequence you want to use to generate the next value.

For example, if you have a sequence named emp_seq:

SELECT emp_seq.NEXTVAL FROM dual;

This will generate the next value in the sequence emp_seq. The dual table is a special dummy table used in Oracle when you need to select a value without referencing any actual table.

3. Important Characteristics of NEXTVAL

• Uniqueness: The primary purpose of a sequence is to generate unique values. NEXTVAL increments the sequence’s current value by the specified increment (default is 1) and returns the new value.
• Non-reversible: Once NEXTVAL is used, the sequence value cannot be rolled back, meaning that even if a transaction is rolled back, the sequence number will not be "un-generated."
• No Duplicates: Each call to NEXTVAL results in a new unique value, which helps ensure that values are unique even if multiple transactions are happening simultaneously.
• Atomicity: Oracle ensures that each call to NEXTVAL returns a unique number, even when multiple users or sessions are accessing the sequence at the same time.

4. Common Use Cases for NEXTVAL

• Primary Keys: The most common use case is generating unique primary keys for records. For example, when inserting data into a table, NEXTVAL can be used to automatically assign a unique value to a column (such as an ID).

Example:

INSERT INTO employees (employee_id, first_name, last_name)

VALUES (emp_seq.NEXTVAL, 'John', 'Doe');

• Order Numbers: Sequences are also used for generating unique order numbers or invoice numbers in applications where each order must have a unique identifier.
• Tracking Numbers: For systems requiring sequential tracking numbers (like ticket numbers, order numbers, etc.), NEXTVAL can generate the number to ensure consistency.

5. Sequence Properties and Behavior

Here are some important points to keep in mind when using sequences with NEXTVAL:

• Increment: Sequences can be set to increment by a value other than 1. For example, you can increment by 5, 10, or any other integer. If no value is specified, the default is 1.

Example of creating a sequence with an increment of 5:

CREATE SEQUENCE my_seq

START WITH 1

INCREMENT BY 5;

• Cyclic Sequences: If the sequence is defined with the CYCLE option, once the sequence reaches its maximum value, it will restart from its minimum value.

Example of creating a sequence that cycles:

CREATE SEQUENCE my_seq

START WITH 1

INCREMENT BY 1

MAXVALUE 10

CYCLE;

• Sequence and CURRVAL: While NEXTVAL generates the next value of the sequence, CURRVAL returns the current value (the last value generated by NEXTVAL). Note that CURRVAL can only be used after NEXTVAL has been called in the session.

Example:

-- Get the next value

SELECT emp_seq.NEXTVAL FROM dual;

 

-- Get the current value

SELECT emp_seq.CURRVAL FROM dual;

• Cache: Sequences in Oracle can be cached to improve performance. When a sequence is cached, Oracle pre-allocates a set of sequence numbers in memory to avoid disk I/O when requesting a new sequence value. However, this means that if the database crashes, numbers that were cached but not yet used may be lost.

Example:

CREATE SEQUENCE my_seq

START WITH 1

INCREMENT BY 1

CACHE 20;

6. Handling Sequences in Transactions

• Atomicity of Sequence Numbers: Sequences ensure that their numbers are unique and that no two transactions can generate the same number. However, if a transaction rolls back, the sequence number generated for that transaction will not be returned to the sequence pool.

For example:

-- Transaction 1

SELECT my_seq.NEXTVAL FROM dual;  -- Returns 1

INSERT INTO my_table VALUES (my_seq.NEXTVAL, 'Sample');  -- Insert with the number 1

 

-- Transaction 2 rolls back

ROLLBACK;  -- The number 1 will not be reused by another transaction

 

-- Transaction 3

SELECT my_seq.NEXTVAL FROM dual;  -- Will return 2, not 1

7. Optimizing Sequence Performance

To improve the performance of sequences, particularly in environments with high transaction rates, consider the following:

• Caching: Use the CACHE option to reduce the overhead of fetching sequence values from disk.
• Minimizing Gaps: If you need to avoid gaps in sequence numbers (e.g., for invoice numbers), be aware that rolling back a transaction may leave gaps in the sequence. This behavior is inherent to how sequences work and cannot be changed.

Conclusion

The NEXTVAL function is a powerful tool in Oracle for generating unique, sequential numbers. Whether for primary keys, order numbers, or other unique identifiers, it plays a crucial role in ensuring consistency and integrity in database applications. The ability to customize sequences (through options like START WITH, INCREMENT BY, CACHE, and CYCLE) provides great flexibility to meet various needs.