75_Combining_queries_together.asciidoc

Combining queries together

Real world search requests are never simple; they search multiple fields with various input text, and filter based on an array of criteria. To build sophisticated search, you will need a way to combine multiple queries together into a single search request.

To do that, you can use the bool query. This query combines multiple queries together in user-defined boolean combinations. This query accepts the following parameters:

must

Clauses that must match for the document to be included.

must_not

Clauses that must not match for the document to be included.

should

If these clauses match, they increase the _score; otherwise, they have no effect. They are simply used to refine the relevance score for each document.

filter

Clauses that must match, but are run in non-scoring, filtering mode. These clauses do not contribute to the score, instead they simply include/exclude documents based on their criteria.

Because this is the first query we’ve seen that contains other queries, we need to talk about how scores are combined. Each sub-query clause will individually calculate a relevance score for the document. Once these scores are calculated, the bool query will merge the scores together and return a single score representing the total score of the boolean operation.

The following query finds documents whose title field matches the query string how to make millions and that are not marked as spam. If any documents are starred or are from 2014 onward, they will rank higher than they would have otherwise. Documents that match both conditions will rank even higher:

{
    "bool": {
        "must":     { "match": { "title": "how to make millions" }},
        "must_not": { "match": { "tag":   "spam" }},
        "should": [
            { "match": { "tag": "starred" }},
            { "range": { "date": { "gte": "2014-01-01" }}}
        ]
    }
}

Tip	If there are no must clauses, at least one should clause has to match. However, if there is at least one must clause, no should clauses are required to match.

Adding a filtering query

If we don’t want the date of the document to affect scoring at all, we can re-arrange the previous example to use a filter clause:

{
    "bool": {
        "must":     { "match": { "title": "how to make millions" }},
        "must_not": { "match": { "tag":   "spam" }},
        "should": [
            { "match": { "tag": "starred" }}
        ],
        "filter": {
          "range": { "date": { "gte": "2014-01-01" }} (1)
        }
    }
}

1. The range query was moved out of the should clause and into a filter clause

By moving the range query into the filter clause, we have converted it into a non-scoring query. It will no longer contribute a score to the document’s relevance ranking. And because it is now a non-scoring query, it can use the variety of optimizations available to filters which should increase performance.

Any query can be used in this manner. Simply move a query into the filter clause of a bool query and it automatically converts to a non-scoring filter.

If you need to filter on many different criteria, the bool query itself can be used as a non-scoring query. Simply place it inside the filter clause and continue building your boolean logic:

{
    "bool": {
        "must":     { "match": { "title": "how to make millions" }},
        "must_not": { "match": { "tag":   "spam" }},
        "should": [
            { "match": { "tag": "starred" }}
        ],
        "filter": {
          "bool": { (1)
              "must": [
                  { "range": { "date": { "gte": "2014-01-01" }}},
                  { "range": { "price": { "lte": 29.99 }}}
              ],
              "must_not": [
                  { "term": { "category": "ebooks" }}
              ]
          }
        }
    }
}

1. By embedding a bool query in the filter clause, we can add boolean logic to our filtering criteria

By mixing and matching where Boolean queries are placed, we can flexibly encode both scoring and filtering logic in our search request.

constant_score Query

Although not used nearly as often as the bool query, the constant_score query is still useful to have in your toolbox. The query applies a static, constant score to all matching documents. It is predominantly used when you want to execute a filter and nothing else (e.g. no scoring queries).

You can use this instead of a bool that only has filter clauses. Performance will be identical, but it may aid in query simplicity/clarity.

{
    "constant_score":   {
        "filter": {
            "term": { "category": "ebooks" } (1)
        }
    }
}

1. A term query is placed inside the constant_score, converting it to a non-scoring filter. This method can be used in place of a bool query which only has a single filter