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 an eviction policy can also drop values. 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.