The goal of the relational model was to hide that implementation detail behind a cleaner interface.
Object-relational mapping (ORM) frameworks like ActiveRecord and Hibernate
reduce the amount of boilerplate code required for this translation layer, but they
can’t completely hide the differences between the two models.
In a relational database, the query optimizer automatically decides which parts of the
query to execute in which order, and which indexes to use.
Schema flexibility in the document model
Most document databases, and the JSON support in relational databases, do not
enforce any schema on the data in documents.
If the data in your application has a document-like structure (i.e., a tree of one-to many
relationships, where typically the entire tree is loaded at once), then it’s probably a good idea to use a document model. The relational technique of shredding—
splitting a document-like structure into multiple tables can lead to cumbersome schemas and unnecessarily complicated application code.
Schema-on-read is similar to dynamic (runtime) type checking in programming languages,
whereas schema-on-write is similar to static (compile-time) type checking.
If your application often needs to access the entire document, there is a performance advantage to this storage locality.
The locality advantage only applies if you need large parts of the document at the
same time.
It seems that relational and document databases are becoming more similar over
time, and that is a good thing
