let's say I insert a doc and it stays in translog without refresh.
then I upsert(actually update it), will this upsert miss the doc due to it is not in the segment or not?
From Making Changes Persistent | Elasticsearch: The Definitive Guide [2.x] | Elastic
When you try to retrieve, update, or delete a document by ID, it first checks the translog for any recent changes before trying to retrieve the document from the relevant segment.
Also, refresh doesn't clear the translog, flush does.