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?

Answer 1

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