Oh dear. Shuffle. On the surface, shuffle is not necessarily a bad thing and may even be expected, but not in this scenario. So what is shuffle and why can it be bad?
Shuffle occurs when a part of a distributed table is moved to a different node during query execution. To do this a hash value is computed using the join columns, the node is then found that has that hash value and the row is then sent to that node for processing. If the tables you’re joining on have different hash distributions, you’d expect this to happen, (hence why it’s not always a bad thing!), but in this case I’ve used the same value to hash both of my tables, so what’s gone wrong?
Click through to see what’s gone wrong.