[BUG][PPL] NPE when running head 10 | stats count() by span(timestamp, 1h)

[joshuali925]

What is the bug?
Getting NPE when calling stats with span

How can one reproduce the bug?
Steps to reproduce the behavior:

1. create test index

POST /test/_doc
{
  "observedTime": "2024-12-02T21:19:50Z"
}

2. run query to see error

POST /_plugins/_ppl
{
  "query": "source = test | head 10 | stats count() by span(observedTime, 1h)"
}

{
  "error": {
    "reason": "Invalid Query",
    "details": "Cannot invoke \"java.lang.Comparable.compareTo(Object)\" because \"k1\" is null",
    "type": "NullPointerException"
  },
  "status": 400
}

Note that without head 10 it works correctly, maybe some issue with span when running in memory?

What is the expected behavior?
should not have NPE and should return results correctly

{
  "schema": [
    {
      "name": "count()",
      "type": "integer"
    },
    {
      "name": "span(observedTime,1h)",
      "type": "timestamp"
    }
  ],
  "datarows": [
    [
      1,
      "2024-12-02 21:00:00"
    ]
  ],
  "total": 1,
  "size": 1
}

What is your host/environment?

• OS: [e.g. iOS]
• Version [e.g. 22] 2.17
• Plugins

Do you have any screenshots?
If applicable, add screenshots to help explain your problem.

Do you have any additional context?
Add any other context about the problem.

[joshuali925]

Took a look, it's already fixed by #3079

This was working in 2.15, not working in 2.17/2.18, and should be working again in 2.19