When to use Haystack/ElasticSearch vs Django's ORM

To make things clear I guess you're talking about querying Elasticsearch via Haystack without later instantiating any objects for your search results with data from you database.

Some points to consider besides the points mentioned in the other post:

• A search engine like Elasticsearch is highly optimized when dealing with full-text searches (When doing something with SQL it highly depends on the database/engine you are using)

• Queries that are involving a lot of relations/joins will most like be easier to handle with the ORM, but on the other hand you can eg save data from foreign-key relations in a denormalized fashion when using ES which could give you a performance boost. Of course you can denormalize your database tables as well but this is quite often considered as a bad practice as long as you know what you are doing, eg when solving a performance bottleneck.

• ES is somehow quite easy to scale while scaling your SQL DB might be more complicated.

• Most likely this is a decision that depends very much on your use case, the amount of data to process and the queries you are intending to run. So the best thing of course is - as always - to do some benchmarking yourself and compare this two solutions. But don't do any premature optimisations as one big advantage of the ORM is to keep things simple - you don't have to care much about the integrity of your data and maintain an additional system.