[Sqlalchemy-tickets] Issue #3518: (PostgreSQL) Ordering a DISTINCT ON query adds columns to SELECT (zzzeek/sqlalchemy)

[Thijs D. <iss...@bi...>]

New issue 3518: (PostgreSQL) Ordering a DISTINCT ON query adds columns to SELECT
https://bitbucket.org/zzzeek/sqlalchemy/issues/3518/postgresql-ordering-a-distinct-on-query

Thijs Damsma:

When a distinct query is ordered, the columns on which are sorted are added to the SELECT statement, see example code:

```
#!python

from sqlalchemy.orm.session import sessionmaker
from sqlalchemy.dialects import postgresql
from sqlalchemy.ext.declarative import declarative_base
from sqlalchemy import Column, Integer

Base = declarative_base()


class User(Base):
    __tablename__ = 'user'
    id = Column(Integer, primary_key=True)
    att1 = Column(Integer)
    att2 = Column(Integer)


Session = sessionmaker()
sess = Session()

q = sess.query(User.att2).distinct(User.att1)

print(str(q.statement.compile(dialect=postgresql.dialect())))
```
as expected, only user.att2 will be returned:
```
#!sql

SELECT DISTINCT ON ("user".att1) "user".att2 
FROM "user"
```

Now with an order_by statement:
```
#!python

print(str(q.order_by(User.att2, User.id).statement.compile(
    dialect=postgresql.dialect())))
```

not as expected, both user.att2 and user.id will be returned
```
#!sql
SELECT DISTINCT ON ("user".att1) "user".att2, "user".id 
FROM "user" ORDER BY "user".att2, "user".id
```