Suppose you want to fix the sequence associated with the id column of your Users table. This problem can be addressed in a few ways. It is now time to see how to address the out-of-sync problem and solve it once and for all. And this leads to the aforementioned error because an ID must be unique by definition. Specifically, the error happens when the sequence returns a value for the id column that is already in use. The error as it appears in ArctypeĪs you can imagine, the problem lies in the sequence related to the id column that went out of sync. Also, this will never happen when performing UPDATE queries. When you specify no value for the id column and manually launch an INSERT query or let your ORM ( Object-Relational Mapping) do it for you, Postgres will always return a "duplicate key value violates unique constraint" error. The last one is the most common cause and usually occurs because Postgres uses a sequence and automatically updates its value only when you omit the id field or use the DEFAULT keyword in the INSERT query. inserting a new record into a table by manually specifying the id field in the INSERT query.manually setting the wrong value of a sequence with the setval() function.importing many rows with an INSERT script or restoring an extensive database.When do sequences go out of sync?īased on my experience as a senior software developer, there are three reasons why a Postgres sequence can go out of sync. Generally, you should use SERIAL when creating a new table with CREATE TABLE. Similarly, the special type SERIAL initializes an auto-incremental numeric primary key using a sequence behind the scene. You can define a sequence in Postgres with the CREATE SEQUENCE statement. In detail, you can define a sequence with an initial value and let PostgreSQL decrement it for you at each INSERT. The main difference is that Postgres sequences can also be decremental. If you are familiar with MySQL, the result of having a sequence in Postgres is comparable to the AUTO_INCREMENT behavior. In particular, Postgres uses a sequence to generate a progressive number, which usually represents an automatically incremented numeric primary key. The official documentation explains that a sequence is nothing more than a number generator. Delving into the out-of-sync sequence problem in Postgres What is a Postgres sequence? We will learn about sequences, see what circumstances lead to the out-of-sync issue, learn how to diagnose the problem, and finally solve it. In this article, we'll teach you about the out-of-sync issue and show you how to solve it. Postgres even returns a confusing error message when this happens. Specifically, Postgres sequences are prone to go out of sync, preventing you from inserting new rows. Yet, Postgres has some well-known issues, and one of the most annoying ones involves sequences. It's particularly appreciated by the developer community because it supports both SQL and JSON querying, making it is both relational and non-relational compliant. Postgres is one of the most advanced and widely used open-source RDBMS (Relational Database Management Systems) in the world.
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