What is the difference between must and filter in Query DSL in elasticsearch?

Question

I am new to elastic search and I am confused between must and filter. I want to perform an and operation between my terms, so I did this

POST /xyz/_search

{
    "query": {
        "bool": {
            "must": [
                {
                    "term": {
                        "city": "city1"
                    }
                },
                {
                    "term": {
                        "saleType": "sale_type1"
                    }
                }
            ]
        }
    }
}

which gave me the required results matching both the terms, and on using filter like this

POST /xyz/_search

{
    "query": {
        "bool": {
            "must": [
                {
                    "term": {
                        "city": "city1"
                    }
                }
            ],
            "filter": {
                "term": {
                    "saleType": "sale_type1"
                }
            }
        }
    }
}

I get the same result, so when should I use must and when should I use filter? What is the difference?

Solution

must contributes to the score. In filter, the score of the query is ignored.

In both must and filter, the clause(query) must appear in matching documents. This is the reason for getting same results.

You may check this link

Score

The relevance score of each document is represented by a positive floating-point number called the _score. The higher the _score, the more relevant the document.

A query clause generates a _score for each document.

To know how score is calculated, refer this link

Comments

[intiha]

doesnt really answer when the must vs filter will be used. As in when is having a score important for anything>?

[Cardin]

@intiha Use filter for faster searches, because there is no score to compute and no ranking to be done. Scoring becomes important when you care about say, the relevance due to no. of occurrences of your search term, the length of your matched document, or if you want to add boost to your query to uprank matching documents.

[Paul]

@intiha Score is not important if all your queries are boolean. Clauses used in filters may be cached: "Filter clauses are executed in filter context, meaning that scoring is ignored and clauses are considered for caching." Ref: elastic.co/guide/en/elasticsearch/reference/current/…

[moon]

I think it also matters when you have a compound query where the score of inner query is used to calculate overall score. Same for must_not.