The other example to watch out for is like the first but has to do with joining two tables, both of which are HASH, but with different distribution keys. If you think about that for a second, you can probably already visualize why this might cause a data shuffle. For example, a READING table with a HASH distribution key of ELECTRODE_ID and another table READING_TMP with a HASH distribution key of FREQUENCY_ID can cause a data shuffle. If you executed a query that joined those two tables together, the data may not exist on the same node. In that case, the data from one would need to be moved/shuffled to the one on which the query is executed. JOINs and aggregates have not been fully explained up to now but are covered later in this chapter in the Querying data section. Skip ahead if you want to learn about them now, and then come back here.