Add database_reviews docs section describing final query plans

This comes up all the time in code review and having it spelled out more
clearly in the docs may make it easier for newer contributors to know
about this requirement. Currently it's only vaguely described on this page.

doc/development/database_review.md

@@ -193,6 +193,13 @@ Include in the MR description:
   plan _before_ and _after_ the change. This helps spot differences quickly.
 - Include data that shows the performance improvement, preferably in
   the form of a benchmark.
+- When evaluating a query plan, we need the final query to be
+  executed against the database. We don't need to analyze the intermediate
+  queries returned as `ActiveRecord::Relation` from finders and scopes.
+  PostgreSQL query plans are dependent on all the final parameters,
+  including limits and other things that may be added before final execution.
+  One way to be sure of the actual query executed is to check
+  `log/development.log`.
 
 #### Preparation when adding foreign keys to existing tables