[anonetal]

Rewrite phase is also supposed to "flatten subqueries/views" (cf. "Extensible/rule based query rewrite optimization in Starburst"; SIGMOD 92) and can be fairly complex. PostgreSQL doesn't do it to the same extent as DB2, but there is certainly some complex logic involved there.

[anarazel]

That paper's definition of rewriting isn't the same as postgres' (they even say that: "The work presented here should not be confused with the query rewrite facility of POSTGRES [SJGP90]."). Flattening subqueries and views happens during planning in postgres.

Source: I'm a PostgreSQL Developer.