WT-level validation for column index was added in SERVER-65484 .

MongoDB-level validation must be added next. A threshold issue is how to reuse our existing index-validation code, despite the fact that it uses KeyStrings (which column index does not use, but rather std::strings). There was an attempt to generalize this previously in SERVER-67792 .

Also ian.boros@mongodb.com has commented:

"The mongodb-level validation will probably be some kind of algorithm where we walk the index, hash a bunch of things, walk the collection, hash a bunch of things and then compare them."

"In addition to regular collection validation, we may want to add some extra checks in debug builds. This will depend on exactly how normal collection validation works. Presumably normal validation will walk the row store, and open cursors on each column. Then for each document, generate all keys, and check that they appear in the columns as expected.
A "debug mode validation" could use all of the "leaf" columns to re-materialize the document, which will give us a little more coverage for the object re-materialization logic."