I am currently working on building a new site which is migrating from an existing one.  Front-end work mostly complete, now to creating the admin.

The Problem

Today, a really quite annoying “feature” of PostgreSQL raises its head and takes up far too much time to resolve.  For some background, the data is from an old site built in PHP & MySQL, being migrated to Django & PostgreSQL. This has been working fine, the frontend works and that’s great.

To maintain the old database id’s we inserted everything en-mass into the new database. This in itself was not the problem.  The problem arose when starting to adding new row and are expected auto-increment to the end.

Just a simple query, nothing overly complicated here.

INSERT INTO table (title) VALUES ('my_title');

However, instead of actually inserting into a new row at the end, it started working from id 1. This throws up errors whenever the id already existed (this differs from MySQL). Worse still, on some occasions started to delete items from the end of the database.

The Solution

So, what’s going on? I was initially expecting it to be a problem with my code although but, after plenty of testing, that wasn’t the case.

The primary key wasn’t updating the auto-increment value as expected. Here was the simple fix (change field names where necessary). This will reset the internal counter to the next available number.

SELECT setval('"table_id_seq"', coalesce(max("id"), 1), max("id") IS NOT null) FROM "table";

If you are using Django  you can generate the SQL by running, it does not run the automatically but you can pipe the response directly into the psql tool.

python manage.py sqlsequencereset appname

Very simple, but this does not follow the MySQL (less control over increments and automatically finding the next available number). Then again SQLite also does things differently and no doubt others as well.

It really pays to be aware of some of the fundamental, often unexpected, differences between databases and frameworks. Embrace change, don’t fear it. Do your research, is it better?, does it make for a logical change? But, don’t embrace unnecessarily.