# Rescore Feature

The rescore feature allows te rescore a document returned by a query based
on a secondary algorithm. Rescoring is commonly used if a scoring algorithm
is too costly to be executed across the entire document set but efficient enough
to be executed on the Top-K documents scored by a faster retrieval method. Rescoring
can help to improve precision by reordering a larger Top-K window than actually
returned to the user. Typically is it executed on a window between 100 and 500 documents
while the actual result window requested by the user remains the same.

# Query Rescorer

The `query` rescorer executes a secondary query only on the Top-K results of the actual
user query and rescores the documents based on a linear combination of the user query's score
and the score of the `rescore_query`. This allows to execute any exposed query as a
`rescore_query` and supports a `query_weight` as well as a `rescore_query_weight` to weight the
factors of the linear combination.

# Rescore API

The `rescore` request is defined along side the query part in the json request:

```json
curl -s -XPOST 'localhost:9200/_search' -d {
  "query" : {
    "match" : {
      "field1" : {
        "query" : "the quick brown",
        "type" : "boolean",
        "operator" : "OR"
      }
    }
  },
  "rescore" : {
    "window_size" : 50,
    "query" : {
      "rescore_query" : {
        "match" : {
          "field1" : {
            "query" : "the quick brown",
            "type" : "phrase",
            "slop" : 2
          }
        }
      },
      "query_weight" : 0.7,
      "rescore_query_weight" : 1.2
    }
  }
}
```

Each `rescore` request is executed on a per-shard basis within the same roundtrip. Currently the rescore API
has only one implementation (the `query` rescorer) which modifies the result set in-place. Future developments
could include dedicated rescore results if needed by the implemenation ie. a pair-wise reranker.
*Note:* Only regualr queries are rescored, if the search type is set to `scan` or `count` rescorers are not executed.

Closes #2640