[LUCENE-8996] maxScore is sometimes missing from distributed grouped responses

• Type: Bug
• Status: Closed
• Priority: Minor
• Resolution: Fixed
• Affects Version/s: 5.3
• Fix Version/s: 9.0, 8.4
• Lucene Fields: New, Patch Available
• Assignee: Christine Poerschke
• Reporter: Julien Massenet
• Created: 18/Dec/15 12:35
• Updated: 28/Aug/22 15:51
• Resolved: 09/Dec/19 14:59

Description

This issue occurs when using the grouping feature in distributed mode and sorting by score.

Each group's docList in the response is supposed to contain a maxScore entry that hold the maximum score for that group. Using the current releases, it sometimes happens that this piece of information is not included:

{
  "responseHeader": {
    "status": 0,
    "QTime": 42,
    "params": {
      "sort": "score desc",
      "fl": "id,score",
      "q": "_text_:\"72\"",
      "group.limit": "2",
      "group.field": "group2",
      "group.sort": "score desc",
      "group": "true",
      "wt": "json",
      "fq": "group2:72 OR group2:45"
    }
  },
  "grouped": {
    "group2": {
      "matches": 567,
      "groups": [
        {
          "groupValue": 72,
          "doclist": {
            "numFound": 562,
            "start": 0,
            "maxScore": 2.0378063,
            "docs": [
              {
                "id": "29!26551",
                "score": 2.0378063
              },
              {
                "id": "78!11462",
                "score": 2.0298104
              }
            ]
          }
        },
        {
          "groupValue": 45,
          "doclist": {
            "numFound": 5,
            "start": 0,
            "docs": [
              {
                "id": "72!8569",
                "score": 1.8988966
              },
              {
                "id": "72!14075",
                "score": 1.5191172
              }
            ]
          }
        }
      ]
    }
  }
}

Looking into the issue, it comes from the fact that if a shard does not contain a document from that group, trying to merge its maxScore with real maxScore entries from other shards is invalid (it results in NaN).

I'm attaching a patch containing a fix.

Issue Links

• relates to: LUCENE-9042 Refactor TopGroups.merge tests
• relates to: LUCENE-9010 extend TopGroups.merge test coverage
• links to: GitHub Pull Request #906