To enhance performance, Vantage caches intermediate aggregate storage areas. The size of the aggregate cache is fixed at 1 MB.
If the number of intermediate storage areas exceeds the capacity of the aggregate cache, Vantage pages the least recently used intermediate storage areas to a spool file on disk, which causes a larger number of aggregation phases to occur and impacts performance.
For details on improving aggregate cache usage for an aggregate UDF, including how to determine the maximum number of intermediate storage areas, see the information about CREATE FUNCTION in Teradata Vantage™ - SQL Data Definition Language Detailed Topics, B035-1184.