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The Resource Manager estimates the resources required for queries to run, and then prioritizes them. You can control how the Resource Manager prioritizes query execution in several ways:
Set a resource pool's run-time priority relative to other resource pools.
Change a running query's priority relative to other queries in the same resource pool.
Move a running query to another resource pool.
For each resource pool, you can manage resources that are assigned to queries that are already running. You assign each resource pool a runtime priority of HIGH, MEDIUM, or LOW. These settings determine the amount of runtime resources (such as CPU and I/O bandwidth) assigned to queries in the resource pool when they run. Queries in a resource pool with a HIGH priority are assigned greater runtime resources than those in resource pools with MEDIUM or LOW runtime priorities.
While runtime priority helps to manage resources for the resource pool, there may be instances where you want some flexibility within a resource pool. For instance, you may want to ensure that very short queries run at a high priority, while also ensuring that all other queries run at a medium or low priority.
The Resource Manager allows you this flexibility by letting you set a runtime priority threshold for the resource pool. With this threshold, you specify a time limit (in seconds) by which a query must finish before it is assigned the runtime priority of the resource pool. All queries begin running with a HIGH priority; once a query's duration exceeds the time limit specified in the runtime priority threshold, it is assigned the runtime priority of the resource pool.
You specify runtime priority and runtime priority threshold by setting two resource pool parameters with
CREATE RESOURCE POOL or
ALTER RESOURCE POOL:
RUNTIMEPRIORITY
RUNTIMEPRIORITYTHRESHOLD
CHANGE_CURRENT_STATEMENT_RUNTIME_PRIORITY lets you to change a query's runtime priority. You can change the runtime priority of a query that is already executing.
This function takes two arguments:
The query's transaction ID, obtained from the system table
SESSIONS
The desired priority, one of the following string values: HIGH, MEDIUM, or LOW
Superusers can change the runtime priority of any query to any priority level. The following restrictions apply to other users:
They can only change the runtime priority of their own queries.
They cannot raise the runtime priority of a query to a level higher than that of the resource pools.
Changing a query's runtime priority is a two-step procedure:
Get the query's transaction ID by querying the system table
SESSIONS. For example, the following statement returns information about all running queries:
=> SELECT transaction_id, runtime_priority, transaction_description from SESSIONS;
Run CHANGE_CURRENT_STATEMENT_RUNTIME_PRIORITY, specifying the query's transaction ID and desired runtime priority:
=> SELECT CHANGE_CURRENT_STATEMENT_RUNTIME_PRIORITY(45035996273705748, 'low')
If you are the database administrator, you can move queries to another resource pool mid-execution using the MOVE_STATEMENT_TO_RESOURCE_POOL meta-function.
You might want to use this feature if a single query is using a large amount of resources, preventing smaller queries from executing.
When a query is moved from one resource pool to another, it continues executing, provided the target pool has enough resources to accommodate the incoming query. If sufficient resources cannot be assigned in the target pool on at least one node, Vertica cancels the query and attempts to re-plan the query. If Vertica cannot re-plan the query, the query is canceled indefinitely.
When you successfully move a query to a target resource pool, its resources will be accounted for by the target pool and released on the first pool.
If you move a query to a resource pool with PRIORITY HOLD, Vertica cancels the query and queues it on the target pool. This cancellation remains in effect until you change the PRIORITY or move the query to another pool without PRIORITY HOLD. You can use this option if you want to store long-running queries for later use.
You can view the RESOURCE_ACQUISITIONS or RESOURCE_POOL_STATUS system tables to determine if the target pool can accommodate the query you want to move. Be aware that the system tables may change between the time you query the tables and the time you invoke the MOVE_STATEMENT_TO_RESOURCE_POOL meta-function.
When a query successfully moves from one resource pool to another mid-execution, it executes until the greater of the existing and new RUNTIMECAP is reached. For example, if the RUNTIMECAP on the initial pool is greater than that on the target pool, the query can execute until the initial RUNTIMECAP is reached.
When a query successfully moves from one resource pool to another mid-execution the CPU affinity will change.
To manually move a query from its current resource pool to another resource pool, use the MOVE_STATEMENT_TO_RESOURCE_POOL meta-function. Provide the session id, transaction id, statement id, and target resource pool name, as shown:
=> SELECT MOVE_STATEMENT_TO_RESOURCE_POOL ('v_vmart_node0001.example.-31427:0x82fbm', 45035996273711993, 1, 'my_target_pool');