postgresql should be forced to floats if "Float" given

[Mike Bayer]

i.e.:

session.query(func.stddev_population(somecol, type_=Float))

should return a floating point and not Decimal. Right now you need to use a Numeric since PGNumeric picks up on that, but PGFloat does not.

[Mike Bayer]

made a bigger change to the types API here, the DBAPI coltype can now be passed in. 5f6ed1a3f8bb0b2a724c7f07b98936433a3ef053

[Mike Bayer]

Removing milestone: 0.6.0 (automated comment)