arangosh> db.test.ensureIndex({ type: "hash", fields: [ "a[*]" ] }); { "deduplicate" : true, "fields" : [ "a[*]" ], "id" : "test/18095", "isNewlyCreated" : true, "selectivityEstimate" : 1, "sparse" : false, "type" : "hash", "unique" : false, "code" : 201 } arangosh> db.test.save({ a : [ 1, 2 ] }); { "_id" : "test/18098", "_key" : "18098", "_rev" : "_WgE9Q1m--_" } arangosh> db.test.save({ a : [ 1, 3 ] }); { "_id" : "test/18102", "_key" : "18102", "_rev" : "_WgE9Q1q--_" } arangosh> db.test.save({ a : null }); { "_id" : "test/18105", "_key" : "18105", "_rev" : "_WgE9Q1u--_" }