Boolean Query in Elasticsearch

The difference between must clause and filter clause

The must and filter clauses within a boolean query in Elasticsearch have distinct roles and behaviors:

1. must Clause:

   • Purpose: The must clause is used to specify search criteria that must be met for a document to be included in the search results.
   • Behavior: Queries in the must clause contribute to the scoring of the document. This means that matching documents are not only required to match the criteria specified in the must clause but their relevance score is also influenced by how well they match these criteria.
   • Use Case: Use the must clause for conditions that are essential for the search and where the degree of matching affects the relevance of the document.

2. filter Clause:

   • Purpose: The filter clause is used to apply a filter to the search results without affecting the scoring.
   • Behavior: Queries in the filter clause do not contribute to the scoring of documents. They are used purely to include or exclude documents based on the filter criteria. Documents either match the filter or they don't.
   • Use Case: Use the filter clause for conditions that are binary (a document either meets the condition or it doesn't) and when you do not need the condition to influence the relevance score.

In summary, the must clause affects both the inclusion of documents in the search results and their relevance scoring, while the filter clause affects only the inclusion of documents, without impacting their scoring. The choice between must and filter can also have performance implications, as filters can be cached for faster execution in subsequent queries.