Amazon Redshift Exceeded Concurrent Query Limit

What is Amazon Redshift Exceeded Concurrent Query Limit

Understanding Amazon Redshift

Amazon Redshift is a fully managed, petabyte-scale data warehouse service in the cloud. It is designed to handle large-scale data analytics and is optimized for complex queries on large datasets. Redshift allows you to run queries against exabytes of data in Amazon S3 and offers fast query performance using columnar storage and massively parallel processing (MPP).

Identifying the Symptom

When working with Amazon Redshift, you might encounter an error indicating that the concurrent query limit has been exceeded. This typically manifests as a failure to execute new queries, with error messages suggesting that the current number of active queries has reached the cluster's concurrency limit.

Common Error Message

The error message might look something like this:

ERROR: Exceeded concurrent query limit for the cluster

This message indicates that the cluster cannot accept any more queries until some of the current queries have completed.

Exploring the Issue

The root cause of this issue is that the number of concurrent queries running on your Amazon Redshift cluster has exceeded the maximum allowed limit. Each Redshift cluster has a defined concurrency limit, which is determined by the cluster's node type and size. When this limit is reached, additional queries are queued and cannot be executed until the number of active queries falls below the limit.

Concurrency Limits

Concurrency limits are set based on the cluster's configuration. For example, a dc2.large node type may have a different concurrency limit compared to a ra3.4xlarge node type. Understanding these limits is crucial for optimizing query performance and resource utilization.

Steps to Resolve the Issue

To address the issue of exceeding the concurrent query limit, consider the following steps:

1. Increase Concurrency Limit

If your workload requires more concurrent queries than your current cluster configuration allows, you can increase the concurrency limit by upgrading your cluster to a larger node type or adding more nodes. This can be done through the AWS Management Console or using the AWS CLI:

aws redshift modify-cluster --cluster-identifier my-cluster --node-type ra3.4xlarge --number-of-nodes 4

For more details, refer to the Amazon Redshift Cluster Management Guide.

2. Optimize Queries

Review and optimize your queries to reduce their execution time, which can help free up slots for new queries. Techniques include:

Using query optimization techniques such as predicate pushdown and distribution key selection. Analyzing query performance using the Query Monitoring feature.

3. Use Workload Management (WLM)

Configure Workload Management (WLM) to allocate resources effectively. WLM allows you to define queues with specific concurrency and memory settings. Adjusting these settings can help manage query workloads more efficiently:

ALTER SYSTEM SET wlm_json_configuration='[{"query_concurrency":5,"memory_percent":50}]';

For more information, see the WLM Configuration Guide.

Conclusion

By understanding and managing your Amazon Redshift cluster's concurrency limits, you can ensure that your queries run smoothly and efficiently. Whether by increasing resources, optimizing queries, or configuring WLM, these steps will help you maintain optimal performance and avoid exceeding the concurrent query limit.