[Sqlalchemy-tickets] Issue #3641: Distinct on (postgresql dialect) bug ? (zzzeek/sqlalchemy)

[Mehdi G. <iss...@bi...>]

New issue 3641: Distinct on (postgresql dialect) bug ?
https://bitbucket.org/zzzeek/sqlalchemy/issues/3641/distinct-on-postgresql-dialect-bug

Mehdi GMIRA:

I encountred a bug with the postgres specific "DISTINCT ON" in SQLAlchemy
The real SA query I'm using is more complicated than this, but it sums up to :

```
#!python

query = db.session.query(c1, c2, c3, c4).distinct(c1, c2).order_by(c1, c2, c3)
```
when i compile the query by doing:

```
#!python

query.statement.compile(dialect=postgres.dialect())
```
The actual query is 

```
#!sql

SELECT DISTINCT ON (c1, c2) c1, c2, c3, c4, c1, c2, c3 FROM .... ORDER BY c1, c2, c3
```
As you can see, the columns c1, c2, c3 are repeated. After some investigation, it seems like this is caused by the order_by() part of the query (removing order by removes the repeated columns). 
The weird thing is that when actually running the query (query.all()), I only get the column that I requested, and not the repeated ones as in the compiled statement.
This would have been fine with me. But the problem is it's preventing me from using a union(), because SA thinks I'm requesting more columns on one side of union().

I think It's a bug, but maybe I'm just doing something wrong.