Elasticsearch Query Cache: Optimizing Search Performance

What is Query Cache?

Query cache in Elasticsearch is a feature that stores the results of search queries to improve performance for frequently executed queries. When enabled, Elasticsearch saves the results of filter clauses in memory, allowing subsequent identical queries to retrieve results faster without re-executing the entire search process.

Best Practices

1. Enable query cache selectively for queries that are frequently executed and have a high hit rate.
2. Monitor cache usage and adjust cache size based on your cluster's needs and available memory.
3. Use cache keys to control caching behavior for specific queries.
4. Regularly clear the query cache to prevent stale results, especially after index updates.
5. Consider disabling query cache for queries with rapidly changing data or low reuse potential.

Common Issues or Misuses

1. Over-reliance on query cache, leading to excessive memory usage.
2. Caching queries with low hit rates, which can waste resources.
3. Not clearing the cache after significant index updates, resulting in stale results.
4. Ignoring cache evictions, which can indicate undersized cache or inefficient query patterns.
5. Enabling query cache for all queries without considering their characteristics and frequency.

Additional Information

• Query cache is disabled by default and can be enabled at the index level or for specific queries.
• The cache uses a Least Recently Used (LRU) eviction policy to manage entries when it reaches capacity.
• Query cache is particularly effective for filter clauses in bool queries and aggregations.
• The size of the query cache can be configured using the indices.queries.cache.size setting.
• Cache hit ratio and evictions can be monitored using Elasticsearch's stats API.

Frequently Asked Questions

Q: How do I enable query cache in Elasticsearch?
A: Query cache can be enabled at the index level by setting index.queries.cache.enabled: true in the index settings. For individual queries, you can use the query_cache parameter in the search request.

Q: What types of queries benefit most from query caching?
A: Queries that are frequently executed and have a high hit rate benefit most from caching. Filter clauses in bool queries and certain types of aggregations are particularly well-suited for caching.

Q: How can I monitor query cache performance?
A: You can use Elasticsearch's stats API to monitor cache hit ratio, evictions, and memory usage. The /_stats/query_cache endpoint provides detailed information about query cache performance.

Q: Does query cache work with near real-time (NRT) search?
A: Query cache can work with NRT search, but it's important to manage cache invalidation properly. For rapidly changing data, you may need to clear the cache more frequently or consider disabling it for certain queries.

Q: How does query cache differ from request cache in Elasticsearch?
A: Query cache stores the results of filter clauses, while request cache stores the entire search request and its results. Query cache is more granular and can be reused across different queries that share the same filter clauses.