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, 2 insertions, 0 deletions

diff --git a/test/sql/test_labels.py b/test/sql/test_labels.py
index 535d4dd0b..8c8e9dbed 100644
--- a/test/sql/test_labels.py
+++ b/test/sql/test_labels.py
@@ -805,6 +805,8 @@ class ColExprLabelTest(fixtures.TestBase, AssertsCompiledSQL):
 
     def _fixture(self):
         class SomeColThing(WrapsColumnExpression, ColumnElement):
+            inherit_cache = False
+
             def __init__(self, expression):
                 self.clause = coercions.expect(
                     roles.ExpressionElementRole, expression