Warn when caching is disabled / document

This patch adds new warnings for all elements that
don't indicate their caching behavior, including user-defined
ClauseElement subclasses and third party dialects.
it additionally adds new documentation to discuss an apparent
performance degradation in 1.4 when caching is disabled as a
result in the significant expense incurred by ORM
lazy loaders, which in 1.3 used BakedQuery so were actually
cached.

As a result of adding the warnings, a fair degree of
lesser used SQL expression objects identified that they did not
define caching behavior so would have been producing
``[no key]``, including PostgreSQL constructs ``hstore``
and ``array``.  These have been amended to use inherit
cache where appropriate.  "on conflict" constructs in
PostgreSQL, MySQL, SQLite still explicitly don't generate
a cache key at this time.

The change also adds a test for all constructs via
assert_compile() to assert they will not generate cache
warnings.

Fixes: #7394
Change-Id: I85958affbb99bfad0f5efa21bc8f2a95e7e46981

1 files changed, 16 insertions, 0 deletions

diff --git a/lib/sqlalchemy/sql/coercions.py b/lib/sqlalchemy/sql/coercions.py
index 480d2c680..07da49c4e 100644
--- a/lib/sqlalchemy/sql/coercions.py
+++ b/lib/sqlalchemy/sql/coercions.py
@@ -12,6 +12,7 @@ import re
 from . import operators
 from . import roles
 from . import visitors
+from .base import ExecutableOption
 from .base import Options
 from .traversals import HasCacheKey
 from .visitors import Visitable
@@ -458,6 +459,21 @@ class HasCacheKeyImpl(RoleImpl):
         return element
 
 
+class ExecutableOptionImpl(RoleImpl):
+    __slots__ = ()
+
+    def _implicit_coercions(
+        self, original_element, resolved, argname=None, **kw
+    ):
+        if isinstance(original_element, ExecutableOption):
+            return original_element
+        else:
+            self._raise_for_expected(original_element, argname, resolved)
+
+    def _literal_coercion(self, element, **kw):
+        return element
+
+
 class ExpressionElementImpl(_ColumnCoercions, RoleImpl):
     __slots__ = ()