"Filter then Aggregation" or just "Filter Aggregation"?

Question

I am working on ES recently and I found that I could achieve the almost same result but I have no clear idea as to the DIFFERENCE between these two.

"Filter then Aggregation"

POST kibana_sample_data_flights/_search
{
  "size": 0,
  "query": {
    "constant_score": {
      "filter": {
        "term": {
          "DestCountry": "CA"
        }
      }
    }
  },
  "aggs": {
    "ca_weathers": {
      "terms": { "field": "DestWeather" }
    }
  }
}

"Filter Aggregation"

POST kibana_sample_data_flights/_search
{
  "size": 0,
  "aggs": {
    "ca": {
      "filter": {
        "term": {
          "DestCountry": "CA"
        }
      },
      "aggs": {
        "_weathers": {
           "terms": { "field": "DestWeather" } 
        }
      }
    }
  }
}

My Questions

1. Why there are two similar functions? I believe I am wrong about it but what's the difference then? _{(please do ignore the result format, it's not the question I am asking ;p)}
2. Which is better if I want to filter out the unrelated/unmatched and start the aggregation on lots of documents?

Answer 1

Answer from @Val's comment, I may just quote here for reference:

In option A, the aggregation will be run on ALL documents. In option B, the documents are first filtered and the aggregation will be run only on the selected documents. Say you have 10M documents and the filter select only a 100, it's pretty evident that option B will always be faster.