Скачать или смотреть Understanding the NOT NULL Constraint in PostgreSQL for Identity Columns

Explore why passing `NULL` values to identity columns in PostgreSQL is not allowed, the underlying mechanism, and alternative approaches to resolve this issue.
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Understanding the NOT NULL Constraint in PostgreSQL for Identity Columns

If you are working with PostgreSQL, particularly version 11.10 or later, you might encounter a situation where you want to insert a NULL value into an identity column. This can lead to confusion, especially when your code seems to suggest that a NULL should be a valid entry. Here’s an explanation of why this restriction is in place and how you can effectively work around it.

The Problem: What Happens When You Insert NULL

When you attempt to insert a NULL value into an identity column in PostgreSQL, you may see an error similar to the following:

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Identity Column: An identity column is a column that automatically generates numeric values. This feature is especially useful for primary keys, where every entry needs to have a unique identifier.

NULL Entries: In PostgreSQL, inserting NULL into a column that is defined with NOT NULL will raise a violation error.

As shown in your error message and column definition, PostgreSQL expects a valid numeric entry, and NULL does not meet this requirement.

Why is NULL Not Allowed?

The key aspect to understand here is the DEFAULT behavior in PostgreSQL:

When you define an identity column, PostgreSQL will auto-generate values only if no value or the keyword DEFAULT is provided.

Since NULL is treated as an explicit value, it does not trigger the auto-generation of a new entry.

Column Definition Breakdown

Your column is defined as follows:

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The not null constraint implies that this column cannot accept NULL values, hence the violation when you try to insert NULL.

Solutions: How to Work Around the Null Issue

So, what can you do if you still need to handle identity columns properly? You have a couple of viable options:

1. Omit the Column in the Insert Statement

Instead of specifying NULL for your identity column, you can simply omit it from the column list in your INSERT statement:

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By doing this, PostgreSQL will automatically generate the next value for the identity column.

2. Use the DEFAULT Keyword

If you need to list all columns while still allowing the identity column’s value to be auto-generated, use the DEFAULT keyword:

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This statement instructs PostgreSQL to fill the identity column with the next auto-generated value.

Better Error Handling with GENERATED ALWAYS AS IDENTITY

If you use GENERATED ALWAYS AS IDENTITY, you'll receive more informative error messages when trying to insert directly into the identity column:

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Advanced Option: Use a Trigger for Complex Scenarios

If you require more complex behavior or handling of the identity column under specific conditions, a BEFORE trigger can be a suitable solution. Here’s an implementation example:

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This trigger will ensure that a new value is assigned to mycol anytime a new row is inserted.

Conclusion

Understanding how PostgreSQL handles identity columns and the implications of NULL values is crucial for database management. By incorporating the methods mentioned above, you can overcome the NOT NULL violation and effectively utilize identity columns in your database operations.

By being aware of how to navigate these constraints, you can prevent errors and ensure your application runs smoothly.