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How to Use “Delete by Query” in Elasticsearch

Anastasios Antoniadis

Share on X (Twitter) Share on Facebook Share on Pinterest Share on LinkedInElasticsearch, a powerful open-source search and analytics engine, is known for its ability to perform complex searches and aggregations of textual, numerical, and geospatial data. As data within an Elasticsearch index evolves, there might be scenarios where you need to delete specific documents …

Elasticsearch

Elasticsearch, a powerful open-source search and analytics engine, is known for its ability to perform complex searches and aggregations of textual, numerical, and geospatial data. As data within an Elasticsearch index evolves, there might be scenarios where you need to delete specific documents based on certain criteria rather than deleting one document at a time or wiping an entire index. This is where the “Delete by Query” API comes into play, offering a convenient way to delete documents that match a specific query. This article dives into how to effectively use the “Delete by Query” feature in Elasticsearch, covering prerequisites, execution, and best practices.

Understanding “Delete by Query”

“Delete by Query” allows you to specify a query that matches the documents you want to delete. It essentially combines a search operation based on the query and a bulk delete operation on the results of that query. This operation is particularly useful for cleaning up data, such as removing outdated entries or deleting documents that match specific criteria.

Prerequisites

  • Elasticsearch Cluster: Ensure you have access to an Elasticsearch cluster where you have the necessary permissions to perform delete operations.
  • Index with Documents: The “Delete by Query” operation is performed on an index, so you should have an index populated with documents.
  • Understanding of Query DSL: Familiarity with Elasticsearch’s Query DSL (Domain Specific Language) is essential, as you’ll need to construct a query that matches the documents you intend to delete.

How to Use “Delete by Query”

Step 1: Construct Your Query

First, you need to define the criteria for selecting documents to be deleted. This involves constructing a query using Elasticsearch’s Query DSL. For example, if you want to delete all documents from a “blog_posts” index where the “status” field is “draft”, your query might look like this:

{
  "query": {
    "match": {
      "status": "draft"
    }
  }
}

Step 2: Execute “Delete by Query”

Once your query is defined, you can execute the “Delete by Query” request. This can be done using the Elasticsearch REST API. Assuming your Elasticsearch instance is running locally on the default port, the request would look something like this using curl:

curl -X POST "http://localhost:9200/blog_posts/_delete_by_query" -H "Content-Type: application/json" -d'
{
  "query": {
    "match": {
      "status": "draft"
    }
  }
}'

Replace http://localhost:9200 with the address of your Elasticsearch cluster and blog_posts with the name of your index.

Step 3: Monitor the Task

Depending on the size of your data and the complexity of the query, the “Delete by Query” operation might take some time. Elasticsearch runs this operation as a task. You can monitor the progress of this task through the Tasks API or by specifying the wait_for_completion=false parameter in your request, which makes the request return immediately while the task continues in the background.

Best Practices and Considerations

  • Test Your Query: Before executing “Delete by Query”, test your query with a search request to ensure it matches exactly the documents you intend to delete.
  • Backup Your Data: Always have a backup of your data. Accidental deletion of documents is irreversible.
  • Use with Caution: “Delete by Query” can significantly impact cluster performance, especially for large datasets or complex queries. It’s best used during periods of low cluster load.
  • Concurrency and Version Conflicts: Be aware of potential version conflicts if documents that match the query are being indexed or updated while the delete operation is running.
  • Reindex Instead: For very large datasets, it might be more efficient to reindex the documents you want to keep into a new index and delete the old index.

Conclusion

The “Delete by Query” feature in Elasticsearch is a powerful tool for managing your data, allowing for the bulk deletion of documents that match specific criteria. By carefully constructing your queries and considering the operational impact, you can effectively maintain and clean your Elasticsearch indices. Always proceed with caution, ensuring you have backups and have thoroughly tested your queries to avoid unintended data loss.

Anastasios Antoniadis
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