Elasticsearch Mastering Fragment_Size in Elasticsearch for Optimized Search Results

By Opster Team

Updated: Nov 5, 2023

| 1 min read

Overview

Elasticsearch provides a multitude of functionalities. One such feature is the ability to control the size of the fragments returned by the highlighter through the ‘fragment_size’ parameter. This article delves into the intricacies of ‘fragment_size’ and how to effectively use it to optimize search results.

The ‘fragment_size’ parameter in Elasticsearch is a part of the highlighting feature, which is used to highlight search matches in the response. It defines the length of the fragments in characters. By default, the ‘fragment_size’ is set to 100. This means that the highlighter will return fragments of up to 100 characters each. However, this value can be adjusted according to the specific requirements of your search.

Adjusting the ‘fragment_size’ can have a significant impact on the search results. A smaller ‘fragment_size’ will return more precise results, focusing on the exact match within a smaller context. On the other hand, a larger ‘fragment_size’ will provide a broader context around the search match, which can be useful for understanding the relevance of the match within the document.

Usage & Examples

Here’s an example of how to set the ‘fragment_size’ in a search query:

GET /_search
{
    "query" : {
        "match": { "content": "Elasticsearch" }
    },
    "highlight" : {
        "fields" : {
            "content" : {"fragment_size" : 150}
        }
    }
}

In this example, the ‘fragment_size’ is set to 150, which means the highlighter will return fragments of up to 150 characters each.

It’s important to note that the ‘fragment_size’ is not the only parameter that controls the highlighting feature in Elasticsearch. Other parameters such as ‘number_of_fragments’, ‘fragment_offset’, and ‘order’ can also be used to further customize the search results.

The ‘number_of_fragments’ parameter defines the maximum number of fragments that the highlighter should return. By default, it’s set to 5. The ‘fragment_offset’ parameter defines the start offset for each fragment. By default, it’s set to 0. The ‘order’ parameter defines the order in which the fragments should be returned. By default, it’s set to ‘score’, which means the fragments will be returned in the order of their relevance score.

Here’s an example of how to use these parameters in a search query:

GET /_search
{
    "query" : {
        "match": { "content": "Elasticsearch" }
    },
    "highlight" : {
        "fields" : {
            "content" : {
                "fragment_size" : 150,
                "number_of_fragments" : 3,
                "fragment_offset" : 20,
                "order" : "score"
            }
        }
    }
}

In this example, the ‘fragment_size’ is set to 150, the ‘number_of_fragments’ is set to 3, the ‘fragment_offset’ is set to 20, and the ‘order’ is set to ‘score’.

Conclusion

In conclusion, the ‘fragment_size’ parameter in Elasticsearch is a powerful tool that can be used to optimize the search results. By adjusting the ‘fragment_size’ and other related parameters, you can control the precision and context of the search matches, thereby improving the overall search experience.