How to Use Runtime Fields in Elasticsearch

Runtime fields are beneficial when you need to:

• Add new fields to your documents without reindexing
• Perform on-the-fly calculations or transformations
• Explore and analyze data dynamically
• Reduce index size by moving infrequently accessed fields to runtime

Steps to Use Runtime Fields

1. Define a Runtime Field

   Add a runtime field to your index mapping:

   PUT my-index/_mapping
   {
     "runtime": {
       "full_name": {
         "type": "keyword",
         "script": {
           "source": "emit(doc['first_name'].value + ' ' + doc['last_name'].value)"
         }
       }
     }
   }
   

2. Use Runtime Fields in Queries

   Query the runtime field like any other field:

   GET my-index/_search
   {
     "query": {
       "match": {
         "full_name": "John Doe"
       }
     }
   }
   

3. Use Runtime Fields in Aggregations

   Aggregate on runtime fields:

   GET my-index/_search
   {
     "aggs": {
       "full_names": {
         "terms": {
           "field": "full_name"
         }
       }
     }
   }
   

4. Update Runtime Field Definitions

   Modify the runtime field without reindexing:

   PUT my-index/_mapping
   {
     "runtime": {
       "full_name": {
         "type": "keyword",
         "script": {
           "source": "emit(doc['last_name'].value + ', ' + doc['first_name'].value)"
         }
       }
     }
   }
   

Best Practices and Additional Information

• Use runtime fields for infrequently accessed data to optimize index size
• Consider the performance impact of complex scripts in runtime fields
• Runtime fields can be used in most query and aggregation contexts
• They can be defined at index creation, in mappings, or at query time
• Runtime fields do not affect the underlying document source

Frequently Asked Questions

Q: Can runtime fields be used in sorting?
A: Yes, runtime fields can be used for sorting in search queries, but be aware that this may impact performance for large datasets.

Q: Do runtime fields affect indexing performance?
A: No, runtime fields do not affect indexing performance as they are computed at query time. However, they may impact query performance.

Q: Can I convert a runtime field to a regular indexed field?
A: Yes, you can convert a runtime field to an indexed field by reindexing your data and including the runtime field in the new index mapping.

Q: Are there any limitations to using runtime fields?
A: Runtime fields cannot be used in certain contexts, such as highlighting. They also may have performance implications for complex calculations on large datasets.

Q: Can I use runtime fields with machine learning features in Elasticsearch?
A: Runtime fields can be used with some machine learning features, but support may be limited. It's best to check the documentation for specific ML tasks you want to perform.