什么是Azure SQL DW Query计划中的“计算到控制节点”？

What exactly is "Compute to Control Node" step in Azure SQL DW Query execution plan? Does that mean ADW is moving the data to control node and then performing the JOIN. I understand Shuffle operation which redistributes data among the compute nodes. But I did not get in what situation does the data flow from Compute to Control node for a JOIN.What exactly is "Compute to Control Node" step