You can resolve COUNT(DISTINCT) hotspot issues by using partial-final optimization.

Background information

To resolve the COUNT(DISTINCT) hotspot issues, you can change the aggregation process to two-layer aggregation by adding a scattered layer at which data is scattered based on the distinct keys. This is referred to as partial-final optimization. Blink 2.2.0 and later versions support partial-final optimization. Partial-final optimization is suitable for the following scenarios:
  • The COUNT(DISTINCT) function has been used but the performance requirement of an aggregate node is not met.
  • The aggregate node where the COUNT(DISTINCT) function is executed does not have user-defined aggregate functions (UDAFs).
Note Partial-final optimization divides the aggregation process into two layers, which causes additional network shuffling. Therefore, resources are wasted if the data amount is small.

Optimization method

On the Parameters tab of the Development page, set blink.partialAgg.enabled to true.

After partial-final optimization is enabled, check whether an expand node is included in the topology or whether the aggregation process has two layers.

Sample code