Considerations for using document relationships

Over the years, Elasticsearch has improved a lot in reducing memory pressure by introducing doc_values, which is a little slower than the in-memory data structure, fielddata, but still offer reasonable speed and performance. However, because of the way nested and parent-child documents are stored and searched, you should keep the pros and cons in mind before modeling your data. The following is a comparison of nested versus parent-child types, which is nicely outlined by Zachary Tong in one of his articles:

Nested	Parent-Child
Stored in the same Lucene block as each other, which helps in a faster read/query performance. Reading a nested doc is faster than the equivalent parent/child.	Children are stored separately from the parent, but are routed to the same shard. So parent/children performance is slightly less on read/query than nested.
Updating a single field in a nested document (parent or nested children) forces ES to re-index the entire...