Summary:
Some facts are aggregations of other facts. For example, we may compute how many times each macro is used in each object as a "raw fact":
Dnnn uses macro "psyduck" 6 times.
But we want to present this data in aggregate form, e.g. "order macros by popularity". We can do this at runtime and it probably won't be too awful a query, but we can also aggregate it cheaply:
Macro "psyduck" is used 3920 times across all objects.
...and then do a query like "select macros ordered by usage".
"Aggregate" facts support facts like this. The aggregate facts I've implemented are:
- Count of all objects.
- Count of objects of type X.
- Last time facts were updated.
These clearly fit the "aggregate" facts template well. I'm not 100% sure macros do. We can use this table to answer a question like "What are the most popular macros, ordered by use?" We can also use it to answer a question like "What are the most popular macros in the last 6 months?", if we build a specific fact for that. But we can't use it to answer a question like "What are the most popular macros between times X and Y?". Maybe that's important; maybe not.
This seems like a good fit for at least some types of facts.
I'll de-magic the keys a bit in the next diff.
Test Plan: Ran the engines and got some aggregated facts about other facts.
Reviewers: vrana, btrahan
Reviewed By: vrana
CC: aran
Maniphest Tasks: T1562
Differential Revision: https://secure.phabricator.com/D3089
