Elasticsearch Bucket Selector Aggregation - Syntax, Example, and Tips

The Bucket Selector Aggregation is a pipeline aggregation in Elasticsearch that allows you to filter buckets based on specified criteria. It evaluates a script for each bucket and keeps only those buckets for which the script returns true.

Syntax

{
  "bucket_selector": {
    "buckets_path": {
      "my_var1": "metric1",
      "my_var2": "metric2"
    },
    "script": "params.my_var1 > params.my_var2"
  }
}

For more details, refer to the official Elasticsearch documentation.

Example Usage

Here's an example that selects buckets where the average price is greater than 100:

{
  "aggs": {
    "sales_per_month": {
      "date_histogram": {
        "field": "date",
        "calendar_interval": "month"
      },
      "aggs": {
        "avg_price": {
          "avg": {
            "field": "price"
          }
        },
        "high_value_months": {
          "bucket_selector": {
            "buckets_path": {
              "avg_price": "avg_price"
            },
            "script": "params.avg_price > 100"
          }
        }
      }
    }
  }
}

Common Issues

1. Script errors: Ensure that the script syntax is correct and all referenced variables are properly defined in the buckets_path.
2. Missing buckets: If no buckets meet the criteria, the result may be empty. Consider using a bucket_sort aggregation to handle empty results gracefully.
3. Performance impact: Complex scripts can slow down aggregation performance, especially on large datasets.

Best Practices

1. Keep scripts simple and efficient to minimize performance impact.
2. Use bucket_sort in conjunction with bucket_selector for more advanced filtering and sorting.
3. Consider using min_doc_count in parent aggregations to exclude buckets with insufficient data before applying the selector.
4. Test your bucket selector aggregations on a small dataset before running them on large production data.

Frequently Asked Questions

Q: Can I use multiple conditions in a bucket selector script?
A: Yes, you can use multiple conditions in the script. For example: params.avg_price > 100 && params.total_sales > 1000.

Q: How does bucket selector aggregation affect parent aggregations?
A: Bucket selector filters out buckets from the parent aggregation that don't meet the specified criteria. This means that subsequent aggregations will only see the filtered set of buckets.

Q: Can I use bucket selector with nested aggregations?
A: Yes, you can use bucket selector with nested aggregations. Just ensure that the buckets_path correctly references the metrics from the nested aggregations.

Q: Is it possible to use bucket selector to filter based on the bucket key?
A: Yes, you can access the bucket key using _key in your script. For example: params._key > 5 for numeric keys or params._key.startsWith('A') for string keys.

Q: How can I debug a bucket selector aggregation if it's not producing expected results?
A: You can add a gap_policy parameter set to "skip" to see which buckets are being filtered out. Also, consider using Elasticsearch's Explain API to understand how the aggregation is being executed.