Answer: most of the time if you’re expecting a result set > 1.
If you’re new to FHIR, chances are your queries are returning everything in a resource. This is all fine and good until you encounter a resource that contains attachments.
Examples of attachments are photos and PDF files.
Adding _summary=true to your query parameters will exclude all attachments.
Some FHIR resources that can contain attachments are Patient, RelatedPerson, Consent, DocumentReference and DiagnosticReport.
You can tell whether or not an attribute is included in a summary by looking for the sigma symbol (Σ) in the Flags column of the resource definition in the FHIR documentation.
https://www.hl7.org/fhir/patient.html#resource
The screenshot tells us that while a patient’s name and address are included in summaries, their photo is not.
If you’re expecting a result set of 20 patients or documents, best not to include 20 images or 20 PDF files unless you need them.
---
Sign up to “The Tuesday FHIR Sessions” and receive an email every Tuesday where I go deep on a single FHIR topic.