Bug: outer_join generates LEFT join instead of full outer

[scottcode]

When trying to create a full outer join using the outer_join method on a TableExpr, the resulting SQL is actually a LEFT OUTER JOIN.

iconn = ibis.sql.sqlite.client.SQLiteClient('test_outer_join.sqlite', create=True)

iconn.con.execute("DROP TABLE IF EXISTS test_left;")
iconn.con.execute("CREATE TABLE test_left(id_left integer, value_left integer);")
iconn.con.execute("DROP TABLE IF EXISTS test_right;")
iconn.con.execute("CREATE TABLE test_right(id_right integer, value_right integer);")

left = iconn.table('test_left')
right = iconn.table('test_right')

joined = left.outer_join(right, left.id_left == right.id_right)

print(joined.compile())
# SELECT t0.id_left, t0.value_left, t1.id_right, t1.value_right 
# FROM test_left AS t0 LEFT OUTER JOIN test_right AS t1 ON t0.id_left = t1.id_right

'full outer' in str(joined.compile()).lower()
# False

The root cause of it is that the method in ibis.sql.alchemy._AlchemyTableSet.get_result() uses sqlalchemy.sql.selectable.FromClause.outerjoin() with just result.outerjoin(table, onclause). However, starting in sqlalchemy version 1.1.0 (circa 2016) they added a keyword argument full with default False. To fix this we need to change the following:

Current:

elif jtype is ops.OuterJoin:
    result = result.outerjoin(table, onclause)

Proposed fix:

elif jtype is ops.OuterJoin:
    result = result.outerjoin(table, onclause, full=True)

[scottcode]

I submitted my fix. I'm new to contributing to projects like this, so if there are any formalities or technicalities that I need to tweak to make this contribution possible, please let me know. I am open to guidance. Thanks to the community for how awesome ibis already is, and I hope to become part of your community.

[scottcode]

Fixed by PR #1788