How does the context influence the bool query?

[AndreKR]

In my index, there is a document that has ident_key = a, but none with ident_key = b.

Now this query returns one hit:

GET _search
{
  "query": {
    "bool": {
      "filter": [
        {
          "term": {
            "ident_key": "a"
          }
        }
      ],
      "should": [
        {
          "term": {
            "ident_key": "b"
          }
        }
      ]
    }
  }
}

But this query returns no hits:

GET _search
{
  "filter": { 
    "bool": {
      "filter": [
        {
          "term": {
            "ident_key": "a"
          }
        }
      ],
      "should": [
        {
          "term": {
            "ident_key": "b"
          }
        }
      ]
    }
  }
}

The only difference is the outermost filter instead of query.

I don't understand this behavior. Does a bool query work differently in query context than in filter context (apart from the scoring)?

This is Elasticsearch 2.1.

[AndreKR]

Never mind, it's there... in a box... with a big exclamation mark:

If this query is used in a filter context and it has should clauses then at least one should clause is required to match.