PostgreSQL: When Using UPDATE FROM VALUES, Check Constraint is Not Raised – Understanding the Quirks

Introduction

PostgreSQL is an incredibly powerful and feature-rich relational database management system. It’s widely used in production environments due to its reliability, scalability, and robust feature set. One of the most useful features in PostgreSQL is the ability to perform updates using the `UPDATE FROM VALUES` syntax. However, there’s a catch – when using this syntax, check constraints are not raised. In this article, we’ll dive into the details of why this happens and provide clear instructions on how to work around this limitation.

What is UPDATE FROM VALUES?

The `UPDATE FROM VALUES` syntax is a powerful feature in PostgreSQL that allows you to update rows in a table based on a set of values provided in the `VALUES` clause. This syntax is particularly useful when you need to update multiple rows at once, or when you need to update rows based on complex conditions.

UPDATE target_table
SET column1 = new_value1, column2 = new_value2
FROM (VALUES ('value1', 'value2'), ('value3', 'value4')) AS src (column1, column2)
WHERE target_table.column1 = src.column1;

The Problem – Check Constraints Not Raised

Now, let’s say you have a table with a check constraint defined on one of the columns. For example:

CREATE TABLE example (
    id SERIAL PRIMARY KEY,
    value INTEGER,
    CHECK (value > 0)
);

When you try to update the `value` column using the `UPDATE FROM VALUES` syntax, you might expect the check constraint to be raised if the new value is less than or equal to 0. However, this is not the case:

UPDATE example
SET value = src.new_value
FROM (VALUES (1, -1), (2, 0)) AS src (id, new_value)
WHERE example.id = src.id;

In this example, you would expect the update to fail because the new values (-1 and 0) violate the check constraint. However, the update will succeed, and the check constraint will not be raised.

Why Does This Happen?

The reason why check constraints are not raised when using `UPDATE FROM VALUES` is due to the way PostgreSQL handles the update operation. When you use `UPDATE FROM VALUES`, PostgreSQL creates a temporary table with the values specified in the `VALUES` clause, and then performs the update operation using a join between the temporary table and the target table.

This means that the check constraint is not evaluated during the update operation, because the values are not being inserted or updated directly into the table. Instead, the values are being joined with the existing table data, and then the resulting rows are updated.

Workaround 1 – Use a Subquery

One way to work around this limitation is to use a subquery to perform the update operation. Instead of using `UPDATE FROM VALUES`, you can use a subquery to select the new values and then update the table using those values.

UPDATE example
SET value = sub.new_value
FROM (
    SELECT id, new_value
    FROM (VALUES (1, -1), (2, 0)) AS src (id, new_value)
    WHERE src.new_value > 0
) AS sub
WHERE example.id = sub.id;

In this example, the subquery selects only the rows where `new_value` is greater than 0, and then updates the `value` column using those rows. This ensures that the check constraint is raised if any of the new values violate the constraint.

Workaround 2 – Use a Temporary Table

Another way to work around this limitation is to create a temporary table with the new values, and then update the target table using that temporary table.

CREATE TEMP TABLE temp_update (
    id INTEGER,
    new_value INTEGER
);

INSERT INTO temp_update (id, new_value)
VALUES (1, -1), (2, 0);

UPDATE example
SET value = temp_update.new_value
FROM temp_update
WHERE example.id = temp_update.id AND temp_update.new_value > 0;

DROP TABLE temp_update;

In this example, a temporary table `temp_update` is created to hold the new values. The `INSERT INTO` statement populates the temporary table with the new values, and then the update operation is performed using the temporary table. The `WHERE` clause ensures that only rows with `new_value` greater than 0 are updated, raising the check constraint if any of the new values violate the constraint.

Conclusion

In conclusion, while `UPDATE FROM VALUES` is a powerful feature in PostgreSQL, it has a limitation when it comes to check constraints. However, by using a subquery or a temporary table, you can work around this limitation and ensure that check constraints are raised during update operations.

Remember, it’s essential to carefully evaluate the constraints and rules that apply to your database and adjust your update strategy accordingly. By taking the time to understand the quirks of `UPDATE FROM VALUES`, you can ensure data integrity and maintain a robust and reliable database.

Workaround Comparison

Workaround	Description	Advantages	Disadvantages
Subquery	Use a subquery to select new values and update the table	Easy to implement, no extra overhead	May be slower for large datasets
Temporary Table	Create a temporary table with new values and update the table	Flexible, can handle large datasets	Extra overhead, requires more steps

Best Practices

1. Always evaluate the constraints and rules that apply to your database before performing updates.
2. Use transactions to ensure data consistency and integrity.
3. Test your update strategy thoroughly to ensure it meets your requirements.
4. Consider using a subquery or temporary table to work around the check constraint limitation.

By following these best practices and workarounds, you can ensure that your PostgreSQL database remains robust, reliable, and efficient.

Frequently Asked Question

Get the scoop on PostgreSQL’s quirky behavior when updating with VALUES and constraints!