BUG: Fix discover for sa.TIMESTAMP.

`discover` was returning a bytes dtype for the `sa.TIMESTAMP` instead of
`datetime_`.

That bug was because the SQLAlchemy dialect for MSSQL currently directly imports
`sa.TIMESTAMP`, causing a collision in `odo.backends.sql.revtypes`.

Fix by 1) creating a subclass of `mssql.TIMESTAMP` to use as the key in
`revtypes` so that it does not overwrite `sa.TIMESTAMP` 2) assign that subclass
to the mssql dialect's `'TIMESTAMP'` (using `ischema_names`), so that the
subclass will be returned by the type engine instead of `mssql.TIMESTAMP`.

The added test for the (`sa.TIMESTAMP`, `datetime_`) without the fix applied
would result in this error:
```
a = Bytes(), b = DateTime(tz=None), path = ('.measure', "['ts']", '.ty')
kwargs = {'check_dim': True, 'check_record_order': True}

    @assert_dshape_equal.register(object, object)
    def _base_case(a, b, path=None, **kwargs):
>       assert a == b, '%s != %s\n%s' % (a, b, _fmt_path(path))
E       AssertionError: bytes != datetime
E       path: _.measure['value'].ty
```

This patch includes checks for the other types besides `sa.TIMESTAMP` for better
protection against the general case of failures when using `revtypes` to map
SQLAlchemy types to dtypes.

Those extra cases exposed an issue with `sa.Float(precision=24)`.
That case is commented out to keep the fix of this patch on the `sa.TIMESTAMP` mapping.
(I would have used `pytest.param` to mark it xfail, but pytest for this project needs to be
upgraded first.)

See:
blaze#567
blaze#568
https://bitbucket.org/zzzeek/sqlalchemy/issues/4092/type-problem-with-mssqltimestamp
blaze/blaze#1656

odo/backends/sql.py

@@ -83,6 +83,19 @@
 
 revtypes = dict(map(reversed, types.items()))
 
+# Subclass mssql.TIMESTAMP subclass for use when differentiating between
+# mssql.TIMESTAMP and sa.TIMESTAMP.
+# At the time of this writing, (mssql.TIMESTAMP == sa.TIMESTAMP) is True,
+# which causes a collision when defining the revtypes mappings.
+# 
+# See:
+# https://bitbucket.org/zzzeek/sqlalchemy/issues/4092/type-problem-with-mssqltimestamp
+class MSSQLTimestamp(mssql.TIMESTAMP):
+    pass
+
+# Assign the custom subclass as the type to use instead of `mssql.TIMESTAMP`.
+mssql.base.ischema_names['TIMESTAMP'] = MSSQLTimestamp
+
 revtypes.update({
     sa.DATETIME: datetime_,
     sa.TIMESTAMP: datetime_,
@@ -103,7 +116,7 @@
     mssql.UNIQUEIDENTIFIER: string,
     # The SQL Server TIMESTAMP value doesn't correspond to the ISO Standard
     # It is instead just a binary(8) value with no relation to dates or times
-    mssql.TIMESTAMP: bytes_,
+    MSSQLTimestamp: bytes_,
 })
 
 # interval types are special cased in discover_typeengine so remove them from

odo/backends/tests/test_sql.py

@@ -8,7 +8,20 @@
 from functools import partial
 
 import datashape
-from datashape import discover, dshape, float32, float64, Option
+from datashape import (
+    date_,
+    datetime_,
+    discover,
+    dshape,
+    int_,
+    int64,
+    float32,
+    float64,
+    string,
+    var,
+    Option,
+    R,
+)
 from datashape.util.testing import assert_dshape_equal
 import numpy as np
 import pandas as pd
@@ -279,6 +292,28 @@ def test_discover_oracle_intervals(freq):
     assert discover(t) == dshape('var * {dur: ?timedelta[unit="%s"]}' % freq)
 
 
+@pytest.mark.parametrize(
+    'typ,dtype', (
+        (sa.DATETIME, datetime_),
+        (sa.TIMESTAMP, datetime_),
+        (sa.FLOAT, float64),
+        (sa.DATE, date_),
+        (sa.BIGINT, int64),
+        (sa.INTEGER, int_),
+        (sa.BIGINT, int64),
+        (sa.types.NullType, string),
+        (sa.REAL, float32),
+        (sa.Float, float64),
+        # sa.Float(precision=24), float32 (reason="Currently returns float64")
+        (sa.Float(precision=53), float64),
+    ),
+)
+def test_types(typ, dtype):
+    expected = var * R['value': Option(dtype)]
+    t = sa.Table('t', sa.MetaData(), sa.Column('value', typ))
+    assert_dshape_equal(discover(t), expected)
+
+
 def test_mssql_types():
     typ = sa.dialects.mssql.BIT()
     t = sa.Table('t', sa.MetaData(), sa.Column('bit', typ))
@@ -296,7 +331,6 @@ def test_mssql_types():
     t = sa.Table('t', sa.MetaData(), sa.Column('uuid', typ))
     assert_dshape_equal(discover(t), dshape('var * {uuid: ?string}'))
 
-
 def test_create_from_datashape():
     engine = sa.create_engine('sqlite:///:memory:')
     ds = dshape('''{bank: var * {name: string, amount: int},