Amazon Redshift
Database Developer Guide (API Version 2012-12-01)
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Defining Query Queues

When users run queries in Amazon Redshift, the queries are routed to query queues. Each query queue contains a number of query slots. Each queue is allocated a portion of the cluster's available memory and CPU capacity. A queue's memory and resources are divided among the queue's query slots. You can configure WLM properties for each query queue to specify the way that memory is allocated among slots, how queries can be routed to specific queues at run time, and when to cancel long-running queries. You can also use the wlm_slot_count parameter, which is separate from the WLM properties, to temporarily enable queries to use more memory by allocating multiple slots.

By default, Amazon Redshift configures the following query queues:

  • One superuser queue.

    The superuser queue is reserved for superusers only and it can't be configured. You should only use this queue when you need to run queries that affect the system or for troubleshooting purposes. For example, use this queue when you need to cancel a user's long-running query or to add users to the database. You should not use it to perform routine queries. The queue does not appear in the console, but it does appear in the system tables in the database as the fifth queue. To run a query in the superuser queue, a user must be logged in as a superuser, and must run the query using the predefined superuser query group.

  • One default user queue.

    The default queue is initially configured to run five queries concurrently. You can change the concurrency, timeout, and memory allocation properties for the default queue, but you cannot specify user groups or query groups. The default queue must be the last queue in the WLM configuration. Any queries that are not routed to other queues run in the default queue.

Query queues are defined in the WLM configuration. The WLM configuration is an editable parameter (wlm_json_configuration) in a parameter group, which can be associated with one or more clusters. For more information, see Modifying the WLM Configuration.

You can add additional query queues to the default WLM configuration, up to a total of eight user queues. You can configure the following for each query queue:

  • Concurrency level

  • User groups

  • Query groups

  • WLM memory percent to use

  • WLM timeout

Concurrency Level

Queries in a queue run concurrently until they reach the concurrency level defined for that queue. Subsequent queries then wait in the queue. Each queue can be configured to run up to 50 queries concurrently. The maximum total concurrency level for all user-defined queues is 50. The limit includes the default queue, but does not include the reserved Superuser queue, Amazon Redshift allocates, by default, an equal, fixed share of available memory to each queue, and an equal, fixed share of a queue's memory to each query slot in the queue. The proportion of memory allocated to each queue is defined in the WLM configuration using the memory_percent_to_use property. At run time, you can temporarily override the amount of memory assigned to a query by setting the wlm_query_slot_count parameter to specify the number of slots allocated to the query.

By default, WLM queues have a concurrency level of 5. Your workload might benefit from a higher concurrency level in certain cases, such as the following:

  • If many small queries are forced to wait for long-running queries, create a separate queue with a higher concurrency level and assign the smaller queries to that queue. A queue with a higher concurrency level has less memory allocated to each query slot, but the smaller queries require less memory.

  • If you have multiple queries that each access data on a single slice, set up a separate WLM queue to execute those queries concurrently. Amazon Redshift will assign concurrent queries to separate slices, which allows multiple queries to execute in parallel on multiple slices. For example, if a query is a simple aggregate with a predicate on the distribution key, the data for the query will be located on a single slice.

As a best practice, we recommend using a concurrency level of 15 or lower. All of the compute nodes in a cluster, and all of the slices on the nodes, participate in parallel query execution. By increasing concurrency, you increase the contention for system resources and limit the overall throughput.

The memory that is allocated to each queue is divided among the query slots in that queue. The amount of memory available to a query is the memory allocated to the query slot in which the query is running, regardless of the number of queries that are actually running concurrently. A query that can run entirely in memory when the concurrency level is 5 might need to write intermediate results to disk if the concurrency level is increased to 20. The additional disk I/O could degrade performance.

If a specific query needs more memory than is allocated to a single query slot, you can increase the available memory by increasing the wlm_query_slot_count parameter. The following example sets wlm_query_slot_count to 10, performs a vacuum, and then resets wlm_query_slot_count to 1.

set wlm_query_slot_count to 10; 
vacuum; 
set wlm_query_slot_count to 1;

For more information, see Improving Query Performance.

User Groups

You can assign a comma-separated list of user groups to a queue. When a member of a listed user group runs a query, that query runs in the corresponding queue. There is no set limit on the number of user groups that can be assigned to a queue.

Query Groups

You can assign a comma-separated list of query groups for each queue. A query group is simply a label. At run time, you can assign the query group label to a series of queries. Any queries that are assigned to a listed query group will run in the corresponding queue. There is no set limit to the number of query groups that can be assigned to a queue.

Wildcards

You can assign user groups and query groups to a queue either individually or by using wildcards. For example, if you add 'dba_*' to the list of user groups for a queue, any query that is run by a user with a user name that begins with 'dba_' is assigned to that queue. Wildcards are disabled by default.

WLM Memory Percent to Use

To specify the amount of available memory that is allocated to a query, you can set the WLM Memory Percent to Use parameter. By default, each user-defined queue is allocated an equal portion of the memory that is available for user-defined queries. For example, if you have four user-defined queues, each queue is allocated 25 percent of the available memory. The superuser queue has its own allocated memory and cannot be modified. To change the allocation, you assign an integer percentage of memory to each queue, up to a total of 100 percent. Any unallocated memory is managed by Amazon Redshift and can be temporarily given to a queue if the queue requests additional memory for processing.

For example, if you configure four queues, you can allocate memory as follows: 20 percent, 30 percent, 15 percent, 15 percent. The remaining 20 percent is unallocated and managed by the service.

WLM Timeout

To limit the amount of time that queries in a given WLM queue are permitted to use, you can set the WLM timeout value for each queue. The timeout parameter specifies the amount of time, in milliseconds, that Amazon Redshift will wait for a query in a queue to execute before canceling the query.

The function of WLM timeout is similar to the statement_timeout configuration parameter, except that, where the statement_timeout configuration parameter applies to the entire cluster, WLM timeout is specific to a single queue in the WLM configuration.