A prefix query is a query type in Elasticsearch that is used to search for documents that contain a term starting with a specified prefix. It is often used for searching fields that contain text that is organized hierarchically or that has a common prefix, such as file paths or product categories.
When a prefix query is executed, Elasticsearch searches for terms that start with the specified prefix in the inverted index. It returns any documents that contain a term with a prefix that matches the specified prefix.
Here’s an example of a prefix query in Elasticsearch:
GET /my_index/_search
{
"query": {
"prefix": {
"category": "books"
}
}
}
In this example, we are searching the `category` field in the `my_index` index for terms that start with the prefix “books”. The prefix query will return any documents that contain a term in the `category` field that starts with the prefix “books”, such as “books/fiction” or “books/non-fiction”.
The prefix query is case-sensitive, meaning that it will only match documents that contain a term with a prefix that matches the specified prefix, including its case. If you need to perform a case-insensitive search, you can use a combination of the prefix query and the lowercase filter to convert the search term to lowercase before searching:
GET /my_index/_search
{
"query": {
"prefix": {
"category.keyword": {
"value": "BOOKS"
}
}
},
"script_fields": {
"category_lower": {
"script": "params['_source']['category'].toLowerCase()"
}
}
}
In this example, we are using the `category.keyword` field, which is not analyzed and is case-sensitive. We are also using a script field to convert the `category` field to lowercase, which can be useful for displaying the search results in a consistent format.
Overall, the prefix query provides a simple and efficient way to search for terms that start with a specified prefix in Elasticsearch, making it well-suited for searching fields that contain hierarchical or categorial data.