Memo

A memo stores information about the last time that a query function for some query Q was executed:

• Typically, it contains the value that was returned from that function, so that we don't have to execute it again.
  • However, this is not always true: some queries don't cache their result values, and values can also be dropped as a result of LRU collection. In those cases, the memo just stores dependency information, which can still be useful to determine if other queries that have Q as a dependency may have changed.
• The revision in which the memo last verified.
• The changed at revision in which the memo's value last changed. (Note that it may be backdated.)
• The minimum durability of the memo's dependencies.
• The complete set of dependencies, if available, or a marker that the memo has an untracked dependency.