Comment by packetized | Hacker News Reader

Comment by packetized | Hacker News Reader

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"Periodically we do full index merges of multiple indexes into weekly indexes. Then weekly into monthly. etc. ... We do this with an internal tool we've developed named (for lack of anything creative) "index rewriter" similar to spark, hadoop, or map reduce just for doing parallel/concurrent scans of Elasticsearch and then writing the data to a new index."

ES 2.3 brings the reindex API [0], which is an absolute godsend. Also, that is a remarkable amount of computronium brought to bear on what I would charitably describe as a moderately-sized dataset. Is the 500ms query time an absolute hard requirement?

And I may be admitting my ignorance here, but there's also this statement:

"However, Elasticsearch doesn't have a way to efficiently tell us how many documents were returned in a given response."

Are you looking for something above & beyond the "hits" value returned in a query response? Or am I missing something? ex:

  {
  "responses": [
    {
      "took": 16,
      "timed_out": false,
      "_shards": {
        "total": 3,
        "successful": 3,
        "failed": 0
      },
      "hits": {
        "total": 38,
        "max_score": null,
        "hits": [
          { ...

[0]: https://www.elastic.co/guide/en/elasticsearch/reference/2.3/...

So, just a clarification, the "hits.total" is the total number of documents that were found. The only way to know the number of documents in the current set ("hits.hits") is to parse the hits and count. But even that shouldn't be too expensive unless I'm missing something here.

Not even that. The number of hits returned has a default, which is 10. For more, you choose (the size parameter indicates how many results to return). There is a maximum, by default 100,000 I think.

If you do want to return all results, you can use the scroll API.

There's no guessing with the results in Elasticsearch if you read the (surprisingly accurate) documentation.

Edit (moved from reply to myself): I guess I should clarify before someone corrects me: you could get fewer results than the size parameter. But in that case the total hits should be less than your size parameter and it should be clear that the number of returned documents is the difference.

No, "hits.total" is the total number of documents that matched, regardless of what you specified as the number of hits to return.

Right, but if you set the size to 10, then you either return 10 or (hits.total - 10) documents. If hits.total is greater than 10, you know you returned 10 documents; otherwise, you returned hits.total documents.

Ok, yeah, I see what you're saying and that makes sense. But, even my response is premature because you generally have to parse the whole response to get "hits.total" anyway. So unless you have written a partial parser (does an open source one exist?) you are going to be parsing the whole document to get even "hits.total".

Based on the bug filed[1], "Return number of documents in the result counts as HTTP header or in JSON," the implication is that including the number of documents in the result in the JSON would be an acceptable solution. If that's true, then they're OK with parsing the JSON.

Although, to be fair, the related issue[2] specifically asks for an HTTP header.

JSON parsing is generally pretty fast, but I can understand it being unnecessary overhead (although in general, there are other more significant opportunities for optimization).

If you've ever implemented a JSON stream reader/writer, then you've done partial JSON parsing. Check out the tool `jq`[2], which will parse partial JSON surprisingly well and fast (and this is just a utility).

[1] https://github.com/elastic/elasticsearch/issues/18312

[2] https://github.com/elastic/elasticsearch/issues/16993

[3] https://stedolan.github.io/jq/

The max is 10,000 in 2.x. It was higher before, I forget exactly.

You're right. It can, however, be configured higher using the `index.max_result_window`[1].

[1] https://www.elastic.co/guide/en/elasticsearch/reference/curr...

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