Document Databases and NoSQL
Document databases are sometimes called aggregate databases because they tend to hold documents that combine information in a single logical unit — an aggregate. You might have a document that includes a TV episode, series, channel, brand, and scheduling and availability information, which is the total set of result data you expect to see when you search an online TV catch‐up service.
Retrieving all information from a single document is easier with a database (no complex joins as in an RDBMS) and is more logical for applications (less complex code).
The world is awash with documents. Documents are important as they are generally created for a high‐value purpose. Unfortunately many of them are tax documents and bills, but that’s totally out of your control. You’re just helping organizations manage the things!
Loosely, a document is any unstructured or tree‐structured piece of information. It could be a recipe (for cheesecake, obviously), financial services trade, PowerPoint file, PDF, plain text, or JSON or XML document.
Although an online store’s orders and the related delivery and payment addresses and order items can be thought of as a tree structure, you may instead want to use a column store for these. This is because the data structures are known up front, and it’s likely they won’t vary and that you’ll want to do column operations over them. Most of the time, a column store is a better fit for this data.
Some NoSQL databases provide the best of both worlds — poly‐structured document storage and fast field (column) operations.
This makes a document database a bit of a catchall. Interestingly, because of its treelike nature, an effective document store is also capable of storing simpler data structures.
A table, for example, can be modeled as a very flat XML document — that is, one with only a single set of elements, and no sub‐element hierarchies. A set of triples (aka subgraph) can be stored within a single document, or across documents, too. The utility of doing so depends, of course, on the indexing and query mechanisms supported. There’s no point storing triples in documents if you can’t query them.