how to check whether a document exists by docID in ArangoDB without getting its real value?

问题

for c in knows
  filter DOCUMENT(c._from)==null || DOCUMENT(c._to)==null
  remove { _key: c._key} IN knows

I wonder if the 'document (docID)' condition is slow, and whether the document exists or not according to the document ID, without obtaining its real value.

来源：https://stackoverflow.com/questions/62669204/how-to-check-whether-a-document-exists-by-docid-in-arangodb-without-getting-its