EXECUTIONPARALLELISM in resource Pool

hsaxena20

Whats does it mean EXECUTIONPARALLELISM? and how we can set it?
Why it is required?

SruthiA

Execution Parallelism Limits the number of threads used to process any single query issued in a resource pool. It can be set using ALTER RESOURCE POOL statement for a particular resource pool. Default is AUTO which is equal to number of cores of your system. It is used to provide more threads to a query so that it can complete quickly. However it is not ideal to increase EXECUTIONPARALLELISM to high value

if your query has a GROUP BY clause and EXECUTIONPARALLELISM is set to 8, to perform GROUP BY operation vertica will use 8 threads. Since it uses many threads it requires more memory. So you need to have enough memory available in that resource pool where the query is running.

The below blog provides a good picture of Vertica's work load management and how concurrency and parallelism effects workloads

https://www.vertica.com/blog/concurrency-workload-management/

hsaxena20

If we can leave it as default for our user-defined pool than there should be not a problem?

SruthiA

Yes. You can leave it at default which is AUTO. there should not be any issue.