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No difference, then explain plan uses the same filtered aggregator in both cases:

{
      "type": "filtered",
      "aggregator": {
        "type": "longSum",
        "name": "a0:sum",
        "fieldName": "session_length"
      },
      "filter": {
        "type": "selector",
        "dimension": "language",
        "value": "en"
      },
      "name": "a0:sum"
    },
    {
      "type": "filtered",
      "aggregator": {
        "type": "count",
        "name": "a0:count"
      },
      "filter": {
        "type": "selector",
        "dimension": "language",
        "value": "en"
      },
      "name": "a0:count"
    }