[Someone]

I think you are using different interpretations of "you".

If you do:

  update aTable
  set loginInfo = loginInfo - 'lastLogin'

where 'loginInfo' is a jsonb column, you, the programmer, need not write out the parts of the data to keep. PostgreSQL still sees this as 'read column, evaluate expression, write result', so it will read and write the entire jsonb value.