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

In Elasticsearch, Query DSL (Domain-Specific Language) is a powerful language for constructing queries, including various query types such as the bool query. In the bool query, the most common clauses are must, should, must_not, and filter. The must and filter clauses are two frequently compared clauses among these, each with distinct characteristics in functionality and performance.

must Clause

The must clause specifies a set of conditions that query results must satisfy. This is similar to the AND operation in SQL. When using the must clause, Elasticsearch calculates the relevance score (_score) for each result and sorts the results based on this score.

Example:

Suppose we have a document collection containing information about users' names and ages. If we want to find users named "John" with an age greater than 30, we can construct the following query:

json
{
  "query": {
    "bool": {
      "must": [
        { "match": { "name": "John" }},
        { "range": { "age": { "gt": 30 }}}
      ]
    }
  }
}

In this query, the must clause ensures that returned documents satisfy both the name "John" and age greater than 30 conditions, and results are sorted based on relevance scores.

filter Clause

Unlike must, the filter clause is used for filtering query results but does not affect the relevance scores of the results (thus having no impact on sorting). Queries using the filter clause are typically faster because Elasticsearch can cache the results of the filters.

Example:

Similarly, to find users meeting the conditions without concern for sorting, we can use the filter clause:

json
{
  "query": {
    "bool": {
      "filter": [
        { "term": { "name.keyword": "John" }},
        { "range": { "age": { "gt": 30 }}}
      ]
    }
  }
}

In this query, using the filter clause returns all users named "John" with an age greater than 30, but all returned results have the same score because no relevance scoring is performed.

Summary

Overall, the must clause is suitable for scenarios where results need to be scored and sorted based on conditions, while the filter clause is suitable for scenarios where only data filtering is required without scoring. In practical applications, the choice of clause depends on specific query requirements and performance considerations.