Chapter 1 - WiredTiger Engine

At location 3:12 there is a statement “With the current WiredTiger engine, reading information from a document requires the full document to be loaded in RAM”. Is this the case if a projection has been used to reduce the number of keys/fields returned on screen or to the application etc?

yes the whole document is read from disk but only the projection is sent over the wire. the exception is when the projected fields are the indexed fields of an index.