How To Create and Manage Sequences in Snowflake

In this tutorial, we will explore how to create and manage sequences in Snowflake. Sequences are schema-level objects that generate unique, sequential numbers, making them particularly useful for generating values for primary keys or any column that requires a unique value.

What is a Sequence in Snowflake?

A sequence in Snowflake is a special schema-level object used to generate unique, sequential numbers. These numbers can be utilized across sessions and statements, including concurrent statements. This makes sequences particularly useful for generating values for primary keys or any column that requires a unique value. Snowflake sequences offer a way to ensure that each entry in a database table has a distinct identifier, which is crucial for data integrity and efficient data management.

1. Creating a Sequence

To create a sequence in Snowflake, you can use the `CREATE SEQUENCE` command. This command allows you to specify various properties of the sequence, such as its starting value, increment interval, and whether the sequence should generate values in order or not (using the `ORDER` or `NOORDER` options). By default, if you do not specify `ORDER` or `NOORDER`, Snowflake uses `NOORDER` to improve performance during concurrent insert operations.

CREATE SEQUENCE my_sequence
 START = 1
 INCREMENT = 1
 NOORDER;

This code creates a sequence named "my_sequence" that starts at 1 and increments by 1 for each new value. The `NOORDER` option is used to improve performance during concurrent insert operations.

2. Altering a Sequence

Once a sequence is created, you can modify its properties using the `ALTER SEQUENCE` command. This command allows you to change the sequence's increment interval and whether it generates values in order or not. However, it's important to note that you cannot change a sequence from `NOORDER` to `ORDER`.

ALTER SEQUENCE my_sequence
 INCREMENT = 2;


This code alters the "my_sequence" sequence to increment by 2 for each new value.

3. Using a Sequence

To use a sequence for generating unique values, you can reference the sequence in your SQL statements using the `NEXTVAL` expression. Each time `NEXTVAL` is called for a sequence, it generates the next number in the sequence.

INSERT INTO my_table (id, data)
VALUES (my_sequence.NEXTVAL, 'Some data');


This code inserts a new row into "my_table", using the next value from "my_sequence" as the id and 'Some data' as the data.

Common Challenges and Solutions

While working with sequences in Snowflake, you might encounter some challenges:

 
• Snowflake guarantees that the values generated by a sequence are unique; however, it does not guarantee a gap-free sequence. This means there might be missing numbers in the sequence of generated values, especially when transactions are rolled back or when there are concurrent insert operations.
 
• For new sequences and columns, Snowflake uses `NOORDER` by default to improve performance. This behavior means that the generated values are not guaranteed to be in increasing order.
 
• You cannot change a sequence from `NOORDER` to `ORDER`. If you need the sequence values to be in order, you should specify `ORDER` when creating the sequence.

Best Practices for Using Sequences

Here are some best practices for using sequences in Snowflake:

 
• Use sequences for columns that require unique values, such as primary keys.
 
• Specify the `NOORDER` option when creating a sequence to improve performance during concurrent insert operations.
 
• Use the `ALTER SEQUENCE` command to modify the properties of a sequence after it has been created.

Further Learning

For further learning on sequences in Snowflake, consider exploring the following topics:

 
• How to use the `DESCRIBE SEQUENCE`, `SHOW SEQUENCES`, and the `SEQUENCES` Information Schema view commands for managing and inspecting sequences.
 
• How to handle gaps in sequence values.
 
• How to use sequences in combination with other SQL commands and functions.

Recap of Sequences in Snowflake

In this tutorial, we've learned how to create, alter, and use sequences in Snowflake. We've also discussed some common challenges and best practices related to sequences. By understanding how to effectively utilize sequences, you can ensure data uniqueness and integrity in your Snowflake database schemas.

 
• Sequences are used to generate unique, sequential numbers in Snowflake.
 
• You can create, alter, and use sequences using the `CREATE SEQUENCE`, `ALTER SEQUENCE`, and `NEXTVAL` commands, respectively.
 
• Sequences guarantee unique values but do not guarantee a gap-free sequence.