Implement generic Double and related fixed types

Added :class:`.Double`, :class:`.DOUBLE`, :class:`.DOUBLE_PRECISION`
datatypes to the base ``sqlalchemy.`` module namespace, for explicit use of
double/double precision as well as generic "double" datatypes. Use
:class:`.Double` for generic support that will resolve to DOUBLE/DOUBLE
PRECISION/FLOAT as needed for different backends.

Implemented DDL and reflection support for ``FLOAT`` datatypes which
include an explicit "binary_precision" value. Using the Oracle-specific
:class:`_oracle.FLOAT` datatype, the new parameter
:paramref:`_oracle.FLOAT.binary_precision` may be specified which will
render Oracle's precision for floating point types directly. This value is
interpreted during reflection. Upon reflecting back a ``FLOAT`` datatype,
the datatype returned is one of :class:`_types.DOUBLE_PRECISION` for a
``FLOAT`` for a precision of 126 (this is also Oracle's default precision
for ``FLOAT``), :class:`_types.REAL` for a precision of 63, and
:class:`_oracle.FLOAT` for a custom precision, as per Oracle documentation.

As part of this change, the generic :paramref:`_sqltypes.Float.precision`
value is explicitly rejected when generating DDL for Oracle, as this
precision cannot be accurately converted to "binary precision"; instead, an
error message encourages the use of
:meth:`_sqltypes.TypeEngine.with_variant` so that Oracle's specific form of
precision may be chosen exactly. This is a backwards-incompatible change in
behavior, as the previous "precision" value was silently ignored for
Oracle.

Fixes: #5465
Closes: #7674
Pull-request: https://github.com/sqlalchemy/sqlalchemy/pull/7674
Pull-request-sha: 5c68419e5aee2e27bf21a8ac9eb5950d196c77e5

Change-Id: I831f4af3ee3b23fde02e8f6393c83e23dd7cd34d

1 files changed, 12 insertions, 0 deletions

diff --git a/test/dialect/postgresql/test_compiler.py b/test/dialect/postgresql/test_compiler.py
index b98d0fac6..98b974ea5 100644
--- a/test/dialect/postgresql/test_compiler.py
+++ b/test/dialect/postgresql/test_compiler.py
@@ -243,6 +243,18 @@ class CompileTest(fixtures.TestBase, AssertsCompiledSQL):
             "CAST(bar AS someschema.somename) AS bar",
         )
 
+    def test_cast_double_pg_double(self):
+        """test #5465:
+
+        test sqlalchemy Double/DOUBLE to PostgreSQL DOUBLE PRECISION
+        """
+        d1 = sqltypes.Double
+
+        stmt = select(cast(column("foo"), d1))
+        self.assert_compile(
+            stmt, "SELECT CAST(foo AS DOUBLE PRECISION) AS foo"
+        )
+
     def test_cast_enum_schema_translate(self):
         """test #6739"""
         e1 = Enum("x", "y", "z", name="somename")