Django Postgres ArrayField vs One-to-Many relationship

Question

For a model in my database I need to store around 300 values for a specific field. What would be the drawbacks, in terms of performance and simplicity in query, if I use Postgres-specific ArrayField instead of a separate table with One-to-Many relationship?

Answer 1

If you use an array field

• The size of each row in your DB is going to be a bit large thus Postgres is going to be using a lot more toast tables (http://www.postgresql.org/docs/9.5/static/storage-toast.html)
• Every time you get the row, unless you specifically use defer (https://docs.djangoproject.com/en/1.9/ref/models/querysets/#defer) the field or otherwise exclude it from the query via only, or values or something, you paying the cost of loading all those values every time you iterate across that row. If that's what you need then so be it.
• Filtering based on values in that array, while possible isn't going to be as nice and the Django ORM doesn't make it as obvious as it does for M2M tables.

If you use M2M

• You can filter more easily on those related values
• Those fields are postponed by default, you can use prefetch_related if you need them and then get fancy if you want only a subset of those values loaded
• Total storage in the DB is going to be slightly higher with M2M because of keys, and extra id fields
• The cost of the joins in this case is completely negligible because of keys.

Personally I'd say go with the M2M tables, but I don't know your specific application. If you're going to be working with a massive amount of data it's likely worth grabbing a representative dataset and testing both methods with it.