elasticsearch中bucket_script聚合父级必须是多桶聚合

场景

业务需求中，在计算人均通话数时，使用聚合的时候使用到了内联以及反内联聚合，当bucket_script聚合作为反内联reverse_nested的子聚和的时候，会报如下错误：

org.elasticsearch.ElasticsearchException: Elasticsearch exception [type=class_cast_exception, reason=org.elasticsearch.search.aggregations.bucket.nested.InternalReverseNested cannot be cast to org.elasticsearch.search.aggregations.InternalMultiBucketAggregation]

该场景中的嵌套聚合中的子聚合如下（父级聚合此处不展示）：

{
	"reverse_nested": {
		"reverse_nested": {},
		"aggregations": {
			"agg_count": {
				"value_count": {
					"field": "taskId.keyword"
				}
			},
			"agg_cardinality_tel": {
				"cardinality": {
					"field": "tel.keyword"
				}
			},
			"agg_bucketScript": {
				"bucket_script": {
					"buckets_path": {
						"userCount": "agg_cardinality_tel",
						"docCount": "agg_count"
					},
					"script": {
						"source": "params.docCount / params.userCount",
						"lang": "painless"
					},
					"gap_policy": "skip"
				}
			}
		}
	}
}

原因分析

bucket_script聚合的是建立在多桶聚合的前提下，即父级聚合必须是多桶聚合，官方文档已对此做过说明。

bucket_selector 等聚合方式的聚合场景亦是如此。

A parent pipeline aggregation which executes a script which can perform per bucket computations on specified metrics in the parent multi-bucket aggregation. The specified metric must be numeric and the script must return a numeric value.

解决方案

将bucket_script聚合与reverse_nested聚合放在同层级，关联path即可。

{
	"reverse_nested": {
		"reverse_nested": {},
		"aggregations": {
			"agg_count": {
				"value_count": {
					"field": "taskId.keyword"
				}
			},
			"agg_cardinality_tel": {
				"cardinality": {
					"field": "tel.keyword"
				}
			}
		}
	},
	"agg_bucketScript": {
		"bucket_script": {
			"buckets_path": {
				"userCount": "reverse_nested>agg_cardinality_tel",
				"docCount": "reverse_nested>agg_count"
			},
			"script": {
				"source": "params.docCount / params.userCount",
				"lang": "painless"
			},
			"gap_policy": "skip"
		}
	}
}