public static IndexType valueOfSynonym(String name) {
    name = name.toUpperCase();

    switch (name) {
      case "KEY":
        return valueOf("PRIMARY_KEY");
      case "RANGE":
        return valueOf("FUNCTIONAL");
    }

    return valueOf(name);
  }
The index type of a primary key index. A primary key index uses the keys in the region itself.
By creating a primary key index, you make the query service aware of the relationship between
the values in the region and the keys in the region and enable the relationship to be used to
optimize the execution of queries. For example, if the values in a region are employee objects
and the keys in the region is the attribute empId on those employees, then you can create a
primary key index on that region with the indexedExpression "empId".
<p>
The type of the indexedExpression for a primary key index can be any object type. Just as in a
Region, the lookup is based on the implementation of the <code>equals</code> and
<code>hashCode</code> methods in the object.

@see QueryService#createIndex(String, IndexType, String, String)
geode-core/src/main/java/org/apache/geode/cache/query/IndexType.java