How is a partition with a key and without a key selected?

A partition with a key is selected based on the key's value, which determines in which part of the data the record will fall. For example, if data is partitioned by date, then the key is the date, and the system directs the record to the corresponding partition.

Without a key, the partition selection is based on other rules, such as round-robin distribution or hashing of other parameters. This can be used when there is no explicit partitioning key, and data is distributed evenly to balance the load.

Thus:

• With a key: the partition is determined by the key's value (e.g., hash or range).
• Without a key: the partition is chosen according to a distribution algorithm (round-robin, random, etc.).