[BUG] Cannot pushdown a nested root as group-by key

[LantaoJin]

Query Information

PPL Command/Query:

source=test | stats count() by rec
source=test | top rec
source=test | dedup rec

Dataset Information

Dataset/Schema Type

• OpenTelemetry (OTEL)
• Simple Schema for Observability (SS4O)
• Open Cybersecurity Schema Framework (OCSF)
• Custom (details below)

Index Mapping

{
  "mappings": {
    "properties": {
      "rec": {
        "type": "nested",
        "properties": {
          "inner": {
            "type": "integer"
          }
        }
      }
    }
  }
}

Sample Data

{"rec":{"inner": 100}}
{"rec":{"inner": 101}}
{"rec":{"inner": 101}}

Bug Description

Issue Summary:
The queries above will try to trigger an aggregation pushdown which the group-by key (bucket) is rec. Since rec is a nested root, OpenSeach DSL returns null if the bucket fields is a nested field. So it will fallback to execute in Calcite EnumerableAggregate which may have performance issue.