postgresql duplicate key violates unique constraint

This article explains that your sequence might be out of sync and that you have to manually bring it back in sync.

An excerpt from the article in case the URL changes:

If you get this message when trying to insert data into a PostgreSQL database:

ERROR:  duplicate key violates unique constraint

That likely means that the primary key sequence in the table you're working with has somehow become out of sync, likely because of a mass import process (or something along those lines). Call it a "bug by design", but it seems that you have to manually reset the a primary key index after restoring from a dump file. At any rate, to see if your values are out of sync, run these two commands:

SELECT MAX(the_primary_key) FROM the_table;   
SELECT nextval('the_primary_key_sequence');

If the first value is higher than the second value, your sequence is out of sync. Back up your PG database (just in case), then run this command:

SELECT setval('the_primary_key_sequence', (SELECT MAX(the_primary_key) FROM the_table)+1);

That will set the sequence to the next available value that's higher than any existing primary key in the sequence.

Intro

I also encountered this problem and the solution proposed by @adamo was basically the right solution. However, I had to invest a lot of time in the details, which is why I am now writing a new answer in order to save this time for others.

Case

My case was as follows: There was a table that was filled with data using an app. Now a new entry had to be inserted manually via SQL. After that the sequence was out of sync and no more records could be inserted via the app.

Solution

As mentioned in the answer from @adamo, the sequence must be synchronized manually. For this purpose the name of the sequence is needed. For Postgres, the name of the sequence can be determined with the command PG_GET_SERIAL_SEQUENCE. Most examples use lower case table names. In my case the tables were created by an ORM middleware (like Hibernate or Entity Framework Core etc.) and their names all started with a capital letter.

In an e-mail from 2004 (link) I got the right hint.

(Let's assume for all examples, that Foo is the table's name and Foo_id the related column.)

Command to get the sequence name:

SELECT PG_GET_SERIAL_SEQUENCE('"Foo"', 'Foo_id');

So, the table name must be in double quotes, surrounded by single quotes.

1. Validate, that the sequence is out-of-sync

SELECT CURRVAL(PG_GET_SERIAL_SEQUENCE('"Foo"', 'Foo_id')) AS "Current Value", MAX("Foo_id") AS "Max Value" FROM "Foo";

When the Current Value is less than Max Value, your sequence is out-of-sync.

2. Correction

SELECT SETVAL((SELECT PG_GET_SERIAL_SEQUENCE('"Foo"', 'Foo_id')), (SELECT (MAX("Foo_id") + 1) FROM "Foo"), FALSE);

For future searchs, use ON CONFLICT DO NOTHING.