1. Teradata Vantage™ - Database Design, B035-1094-0123K, Release 17.20, Chapter 10: Join and Hash Indexes, Section: "Aggregate Join Indexes": This document states, "Aggregate join indexes can be used to satisfy queries that summarize large amounts of data... The Optimizer can use an aggregate join index to satisfy a query if the columns in the SELECT and WHERE clauses of the query are a subset of the columns in the index definition, and if the query aggregates are the same as or can be derived from the aggregates in the index definition." This directly matches the scenario.
2. Teradata Vantage™ - SQL Data Definition Language Detailed Topics, B035-1184-0423K, Release 17.20, Chapter 12: Join and Hash Indexes, Section: "CREATE JOIN INDEX": The documentation explains, "The primary purpose of an aggregate join index is to enhance the performance of decision support queries that frequently perform the same aggregations on the same column or columns." This confirms that an AJI is the correct tool for a frequently performed aggregate query.