Fix DELETE statement for Redshift Dialect

[haleemur]

Redshift's delete statement varies slightly from Postgresql's. See here for documentation.

Basic delete statements have the same syntax.

For instance, the following is valid SQL in both dialects:

DELETE FROM customer_table WHERE customer_table.id > 1000

However, while the following is a valid statement in Postgresql:

DELETE FROM customer_table 
WHERE customer_table.id = order_table.customer_id 
AND order_table.id < 100

It needs to be written for Redshift as:

DELETE FROM customer_table
USING order_table
WHERE customer_table.id = order_table.customer_id
AND order_table.id < 100

SqlAlchemy should be able to build this resultant query with the following Python snippet:

from sqlalchemy import delete, Table, Column, Integer, MetaData
from redshift_sqlalchemy import RedshiftDialect
meta = MetaData()
customer = Table('customer_table', meta, Column('id', Integer, primary_key=True))
order = Table('order_table', meta, Column('id', Integer, primary_key=True), Column('customer_id', Integer)
del_stmt = delete(order).where(order.c.customer_id==customer.c.id).where(order.c.id<100)
print(del_stmt.compile(dialect=RedshiftDialect())