Within the constraints listed in this topic, you can use UDFs anywhere you use the Amazon Redshift built-in scalar functions. For more information, see SQL Functions Reference.
Amazon Redshift UDFs have the following constraints:
UDFs cannot access the network or write to the file system.
The total size of user-installed libraries cannot exceed 100 MB.
The number of UDFs that can run concurrently per cluster is limited to one-fourth of the total concurrency level for the cluster. For example, if the cluster is configured with a concurrency of 15, a maximum of three UDFs can run concurrently. After the limit is reached, UDFs are queued for execution within workload management queues. For more information, see Implementing Workload Management.