Composite aggregation query with bucket_sort does not work properly

[fabiobozzo]

I have an index to store financial transactions:

{
  "mappings": {
    "_doc": {
      "properties": {
        "amount": {
          "type": "long"
        },
        "currencyCode": {
          "type": "keyword"
        },
        "merchantId": {
          "type": "keyword"
        },
        "merchantName": {
          "type": "text"
        },
        "partnerId": {
          "type": "keyword"
        },
        "transactionDate": {
          "type": "date"
        },
        "userId": {
          "type": "keyword"
        }
      }
    }
  }
}

Here's my query:

GET /transactions/_search
{
  "aggs": {
    "date_merchant": {
      "aggs": {
        "amount": {
          "sum": {
            "field": "amount"
          }
        },
        "amount_sort": {
          "bucket_sort": {
            "sort": [
              {
                "amount": {
                  "order": "desc"
                }
              }
            ]
          }
        },
        "top_hit": {
          "top_hits": {
            "_source": {
              "includes": [
                "merchantName",
                "currencyCode"
              ]
            },
            "size": 1
          }
        }
      },
      "composite": {
        "size": 1,
        "sources": [
          {
            "date": {
              "date_histogram": {
                "calendar_interval": "day",
                "field": "transactionDate"
              }
            }
          },
          {
            "merchant": {
              "terms": {
                "field": "merchantId"
              }
            }
          }
        ]
      }
    }
  },
  "query": {
    "bool": {
      "filter": [
        {
          "term": {
            "userId": "AAA"
          }
        },
        {
          "term": {
            "partnerId": "BBB"
          }
        },
        {
          "range": {
            "transactionDate": {
              "gte": "2022-07-01"
            }
          }
        },
        {
          "term": {
            "currencyCode": "EUR"
          }
        }
      ]
    }
  },
  "size": 0
}

Please note the "size": 1 in the composite aggregation.

If I change it to 3 (based on my data)... I get different results!

That means the bucket_sort operation doesn't work on the whole list of buckets, but just on the returned ones (if it's just one, that means it's not going to be sorted at all!)

How can I sort on ALL the buckets instead?