Andy Brownsword explains the importance of selecting an appropriate hash key:
In Synapse SQL Pools, Hash distributions allow us to somewhat control data distribution. Whilst this can deliver performance in the right scenarios, I noted last week that these tables should be monitored to avoid skew.
In this post I want to demonstrate how to identify the skew, and why a different distribution model may be better suited.
One of the key tricks is that your choice of hash key should be approximately unique. There’s no explicit uniqueness constraint, but having a column with high cardinality and relatively little skew leads to a good distribution.