> One kind of significant downside of partial indexes in PostgreSQL is that they don't work with parameters.
Your example is fundamentally different from parameters sent by a client in a parameterized query, i.e. $1, $2, etc. Also while your flawed example will never use a partial index, parameterized queries can and often will do that. With prepared statements (which are a slightly different concept from parameterized queries) the devil will lie in the details, but even those don't really prevent partial indexes from being used since postgres version 9.2, released years ago.
Your example is fundamentally different from parameters sent by a client in a parameterized query, i.e. $1, $2, etc. Also while your flawed example will never use a partial index, parameterized queries can and often will do that. With prepared statements (which are a slightly different concept from parameterized queries) the devil will lie in the details, but even those don't really prevent partial indexes from being used since postgres version 9.2, released years ago.