Vertica Threads

The EXECUTIONPARALLELISM controls the number of threads that a single operator in the query plan may use. For example, if your query is a GROUP BY and EXECUTIONPARALLELISM is set to 8, then the Scan operator that reads data from the projection would use 8 threads, and the GROUP BY operator would use 8 threads. If the "query budget" happened to be very low - as can be the case in misconfigured resource pools (see RESOURCE_POOL_STATUS) - then there might not be enough memory for the operators to actually fan out to that number of threads.

The actual number of threads can be seen in the EXECUTION_ENGINE_PROFILES data for the executed query. For example looking at the rows with counter_name "rows produced", each row in the system table for that counter_name represents a thread. For the example above, you'd see 8 Scan operators on each node and 8 Group By Hash/Pipeline operators on each node.

Btw it's worth doing some performance testing to see if EXECUTIONPARALLELISM of 32 is actually faster for your use case than 8 or 16. In my experience it's often not, unless the queries are very CPU-expensive.

--Sharon

Thanks Sharon, I appreciate the response!