How to diagnose and fix the 428C9 generated_always error code in Postgres.

The 428C9 error code in PostgreSQL is associated with the generated_always error, which occurs when you attempt to insert or update the value of a column that is defined as GENERATED ALWAYS AS IDENTITY. This means the column is set to automatically generate a new value for each row, and manual inserts or updates to this column are not allowed by default.

To diagnose and fix this issue, you should:

  1. Identify the table and column that is causing the issue.
  2. Ensure you are not attempting to manually insert or update the value of a GENERATED ALWAYS AS IDENTITY column.

Here are some examples and sample code to explain and cover the possibilities:

Example 1: Attempting to Insert a Value into a Generated Column

If you have a table like this:

CREATE TABLE employee (
    name TEXT NOT NULL

And you try to insert a value into the id column:

INSERT INTO employee (id, name) VALUES (1, 'John Doe');

This will result in a 428C9 error because you’re trying to insert a value into a column that is set to be generated automatically.

To fix this, you should omit the id column from your insert statement and let PostgreSQL handle the generation of the unique identifier:

INSERT INTO employee (name) VALUES ('John Doe');

Example 2: Attempting to Update a Value in a Generated Column

If you try to update the value of a generated column:

UPDATE employee SET id = 2 WHERE name = 'John Doe';

This will also result in a 428C9 error for the same reason.

To fix this, you should not attempt to update the value of the id column, as it is managed by PostgreSQL:

-- Correct update statement without touching the 'id' column
UPDATE employee SET name = 'Jane Doe' WHERE id = 1;

Example 3: Overriding the Generated Value

In some cases, you might want to override the automatically generated value, for example during data migration. PostgreSQL allows this with the OVERRIDING SYSTEM VALUE clause:

INSERT INTO employee OVERRIDING SYSTEM VALUE (id, name) VALUES (1, 'John Doe');

However, use this clause with caution, as it may interfere with the normal sequence generation and could lead to duplicate key errors.

General Tips

  • Review your SQL statements to ensure you’re not trying to manually specify values for GENERATED ALWAYS AS IDENTITY columns.
  • Familiarize yourself with the PostgreSQL documentation regarding identity columns and the OVERRIDING SYSTEM VALUE clause.
  • When inserting or updating rows in tables with identity columns, omit these columns from your statements unless you have a specific reason to override the system value.

By understanding the role of generated identity columns and adjusting your SQL statements accordingly, you can avoid the 428C9 error and ensure your database operations run smoothly.

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