don't invoke fromclause.c when creating an annotated

The ``aliased()`` constructor calls upon ``__clause_element__()``,
which internally annotates a ``FromClause``, like a subquery.
This became expensive as ``AnnotatedFromClause`` has for
many years called upon ``element.c`` so that the full ``.c``
collection is transferred to the Annotated.

Taking this out proved to be challenging. A straight remove
seemed to not break any tests except for the one that
tested the exact condition.  Nevertheless this seemed
"spooky" so I instead moved the get of ``.c`` to be in a
memoized proxy method.   However, that then exposed
a recursion issue related to loader_criteria; so the
source of that behavior, which was an accidental behavioral
artifact, is now made into an explcicit option that
loader_criteria uses directly.

The accidental behavioral artifact in question is still
kind of strange since I was not able to fully trace out
how it works, but the end result is that fixing the
artifact to be "correct" causes loader_criteria, within
the particular test for #7491, creates a select/
subquery structure with a cycle in it, so compilation fails
with recursion overflow.
The "solution" is to cause the artifact to occur in this
case, which is that the ``AnnotatedFromClause`` will have a
different ``.c`` collection than its element, which is a
subquery.  It's not totally clear how a cycle is generated
when this is not done.

This is commit one of two, which goes through
some hoops to make essentially a one-line change.

The next commit will rework ColumnCollection to optimize
the corresponding_column() method significantly.

Fixes: #8796
Change-Id: Id58ae6554db62139462c11a8be7313a3677456ad

1 files changed, 25 insertions, 5 deletions

diff --git a/lib/sqlalchemy/sql/selectable.py b/lib/sqlalchemy/sql/selectable.py
index 488dfe721..3d3aea3f2 100644
--- a/lib/sqlalchemy/sql/selectable.py
+++ b/lib/sqlalchemy/sql/selectable.py
@@ -6752,8 +6752,28 @@ TextAsFrom = TextualSelect
 
 
 class AnnotatedFromClause(Annotated):
-    def __init__(self, element, values):
-        # force FromClause to generate their internal
-        # collections into __dict__
-        element.c
-        Annotated.__init__(self, element, values)
+    def _copy_internals(self, **kw):
+        super()._copy_internals(**kw)
+        if kw.get("ind_cols_on_fromclause", False):
+            ee = self._Annotated__element  # type: ignore
+
+            self.c = ee.__class__.c.fget(self)  # type: ignore
+
+    @util.ro_memoized_property
+    def c(self) -> ReadOnlyColumnCollection[str, KeyedColumnElement[Any]]:
+        """proxy the .c collection of the underlying FromClause.
+
+        Originally implemented in 2008 as a simple load of the .c collection
+        when the annotated construct was created (see d3621ae961a), in modern
+        SQLAlchemy versions this can be expensive for statements constructed
+        with ORM aliases.   So for #8796 SQLAlchemy 2.0 we instead proxy
+        it, which works just as well.
+
+        Two different use cases seem to require the collection either copied
+        from the underlying one, or unique to this AnnotatedFromClause.
+
+        See test_selectable->test_annotated_corresponding_column
+
+        """
+        ee = self._Annotated__element  # type: ignore
+        return ee.c  # type: ignore