How to retrieve a view column derivation in PostgreSQL or Greenplum
From the docs


  Views in PostgreSQL are implemented using the rule system.


So essentially, they get rewritten to simply SELECT statemetns. Nothing in the catalog stores the column expression. Internally, when you run \d on a view, it calls pg_catalog.pg_get_viewdef. You can't go further from that with the system catalog. You're going to have to parse that query to get to the column expression, perhaps with something like libpg_query.