[Sqlalchemy-tickets] Issue #3652: Clarify unnest example in FunctionElement.alias documentation (zzzeek/sqlalchemy)

[Sebastian B. <iss...@bi...>]

New issue 3652: Clarify unnest example in FunctionElement.alias documentation
https://bitbucket.org/zzzeek/sqlalchemy/issues/3652/clarify-unnest-example-in

Sebastian Bank:

Using `func.unnest` to unpack a postgres array:

```python
from sqlalchemy import Column, Integer, Text, select, column, func
from sqlalchemy.dialects.postgresql import ARRAY
from sqlalchemy.ext.declarative import declarative_base

class Post(declarative_base()):

    __tablename__ = 'post'

    id = Column(Integer, primary_key=True)
    title = Column(Text, nullable=False)
    tags = Column(ARRAY(Text), nullable=False)

query = select([Post.id, column('tag')])\
    .select_from(func.unnest(Post.tags).alias('tag'))

print(query)
```

As wanted, this gives `SELECT post.id, tag FROM post, unnest(post.tags) AS tag` (which is postgres-specific syntax, I guess).

I found the example given in [the documentation of FunctionElement.alias](https://bitbucket.org/zzzeek/sqlalchemy/src/366f97b5617af0d15cfaf594ec5ef0408c70e873/lib/sqlalchemy/sql/functions.py?at=master&fileviewer=file-view-default#functions.py-198) somewhat confusing because the SELECT-clause does not actually use the unpacked data:

```python
stmt = select([column('data')]).select_from(
    func.unnest(Table.data).alias('data_view'))
```

Wouldn't it make more sense like this (following the example above)?

```python
stmt = select([column('item')]).select_from(
    func.unnest(Table.data).alias('item'))
```

Or maybe this is not the right use of `FunctionElement.alias`?