title: Elasticsearch reindex API partial update
body: <p>So, we are in a situation where we need to reindex Elasticsearch documents from one index to another. We're using the reindex API for that. Though sometimes the document already exists in the destination index. Setting <code>version_type: "external"</code> makes it so that the document in the destination index is updated which works great, except that it performs a full update, I'd like it to perform a partial update on that document.
  Something like setting <code>ctx.op = "partial"</code> would be nice but it's apparently not implemented as of today.
  Any alternative ideas for achieving this would be appreciated.</p>
  
  <p>PS: I'd like to avoid to query the source index for every documents and sending them individually to the destination with upsert, for performances reasons it seems that would be quite slow compared to the reindex API.</p>