While working on MDL-36512, it was required to extend that issue to purge_by_* too, since they lack initialising the cache in case of a store otherwise, given the current stores implementation, purge will not take effect in case of a file store.

That fix has rised this issue: more details in http://tracker.moodle.org/browse/MDL-36512?focusedCommentId=188304&page=com.atlassian.jira.plugin.system.issuetabpanels:comment-tabpanel#comment-188304, included some doubts around $aggregate usage in other methods.

At the moment, this issue is probably a cleanup (=> improvement?), being aggregation declared as a super advanced setting.