When you store datasets in an In-Memory Store, a key column is added and each row is assigned a unique key. You determine how this key is derived in the key attribute of <storeto>.

You can assign keys based on the time rows are added or a unique ID algorithm. You can also use an existing column in the dataset as the key or a combination of columns. For example:

See <storeto> for detailed information on the methods to derive keys.

The example in Store Data in Declared In-Memory Stores combines multiple existing columns as the key. The example in Use an In-Memory Store to Store and Load Datasets for MashZone NextGen Analytics uses #unique to derive numeric keys.