[DRUID 29] doubleMean aggregation function behavior

[Vicent Taylor]

When doubleMean aggregate with no data selected, it returns 0. For other aggregation like doubleMax, doubleMin and doubleSum, it returns null.

The later behavior is more aligned to SQL. Could we improve this behavior?

{
  "queryType": "groupBy",
  "aggregations": [
    {
      "type": "filtered",
      "aggregator": {
        "type": "doubleMean",
        "name": "_agg",
        "fieldName": "metric_value"
      },
      "filter": {
        "type": "expression",
        "expression": " 0 "
      }
    }
  ],
  "dataSource": {
    "type": "inline",
    "columnNames": ["metric_name", "metric_value"],
    "columnTypes": ["STRING", "LONG"],
    "rows": [
      ["test", 1],
      ["test", 2],
      ["test", 3],
      ["test", 4],
      ["test", 5]
    ]
  },
  "intervals": [
    "-10000/10000"
  ],
  "granularity": "all",
  "dimensions": [
    "metric_name"
  ],
  "having": {
    "type": "always"
  }
}

[Vicent Taylor]

The function array_quantile return DOUBLE NaN instead of null when the array is empty. This behavior may also need to be improved.

array_quantile(array(null),0.5)

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