Oracle sequences are independent database objects used to generate unique values ??across sessions and transactions, often used for primary keys or unique identifiers. Its core mechanism is to generate a unique value through NEXTVAL increment, and CURRVAL obtains the current value without incrementing. Sequences do not depend on tables or columns, and support custom start values, step sizes and loop behaviors. Common scenarios during use include: 1. Primary key generation; 2. Order number; 3. Batch task ID; 4. Temporary unique ID. Notes include: transaction rollback causes gaps, cache size affects availability, naming specifications and permission control. Compared to UUID or identity columns, sequences are suitable for high concurrency environments, but they need to be traded down based on the needs.

Oracle sequences are database objects designed to generate unique numeric values, often used for auto-incrementing identifiers. They're especially useful when you need guaranteed uniqueness across multiple sessions and transactions without relying on application logic.

What Exactly Is an Oracle Sequence?

A sequence in Oracle is a schema object that generates a series of unique numbers. These numbers are typically used as primary keys or unique identifiers in tables. Unlike identity columns found in some other databases (like MySQL or PostgreSQL), Oracle sequences are standalone objects, meaning they aren't tied directly to a specific table or column.

When you create a sequence, Oracle preloads it into memory by default, which helps speed up access. You can control how the numbers are generated—such as starting point, increment size, whether they should cycle back after reaching a max value, and more.

How Do Sequences Generate Unique Numbers?

Sequences generate numbers using two main operations:

• NEXTVAL : Increments the sequence and returns the next value.
• CURRVAL : Returns the current value without incrementing.

Each time NEXTVAL is called, Oracle ensures that the number returned is unique across all sessions. Even if multiple users call NEXTVAL at the same time, Oracle handles the locking internally so there's no duplication.

For example:

 SELECT my_sequence.NEXTVAL FROM dual;

This will return the next number in the sequence, say 101, even if someone else just got 100 a million seconds before.

You can use these values ??in INSERT statements like this:

 INSERT INTO employees (id, name) VALUES (my_sequence.NEXTVAL, 'John Doe');

One thing to note: if a transaction is rolled back after using a sequence, the sequence number isn't reused. That means gaps can appear in your numbering, but unique is preserved.

Common Use Cases for Sequences

Here are some typical scenarios where sequences come in handy:

• Primary key generation : Especially useful when inserting records into multiple tables that need related IDs.
• Order or invoice numbering : When you need a system-generated number that's not necessarily tied to a physical row ID.
• Batch job IDs : For tracking jobs or processes with unique identifiers.
• Temporary unique IDs : During complex data transformations or intermediate processing steps.

Because sequences are session-independent and fast, they're ideal for high-concurrency environments where many inserts happens simultaneously.

Another benefit is flexibility—you can use the same sequence for multiple tables, or have different sequences for different tables depending on your design needs.

Gotchas and Best Practices

While sequences are powerful, there are a few things to keep in mind:

• No reuse on rollback : As mentioned earlier, if a transaction fails, the sequence value is lost, creating a gap.
• Cache size matters : By default, Oracle caches 20 values. If the database crashes, those cached values ??are lost. You can increase or disable caching depending on your tolerance for gaps.
• Naming conventions help : Using consistent names like table_name_seq makes it easier to track what each sequence is used for.
• Permissions : Make sure users or applications have the right privileges to access the sequence.

If you're designing a new system, consider whether you really need a sequence or if you can rely on other mechanisms like UUIDs or identity columns (available in Oracle 12c and later).

So that's how Oracle sequences work and why they're commonly used. Not too complicated, but easy to misuse if you don't understand how they behave under load or failure conditions.

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