In the relational world, data is often divided into multiple tables and is linked using foreign keys. To get the data, a join is used to combine data from one or more tables. But in the NoSQL world, data is usually denormalized and stored as one big document. However, it is often advantageous to store these documents separately. Data in Elasticsearch is immutable. An update to an existing document means fetching the old document, applying the change, and re-indexing it as a new document. The update is an expensive operation. If possible, we have to keep the updates to a minimum.
For example, a blog article can have one or more comments, and an order can have one or more line items. If we can separate the article and comment documents, we don't have to update the article when there is a new comment. Elasticsearch provides...
