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, 8 insertions, 0 deletions

diff --git a/test/sql/test_operators.py b/test/sql/test_operators.py
index 831b75e7e..6e943d236 100644
--- a/test/sql/test_operators.py
+++ b/test/sql/test_operators.py
@@ -656,6 +656,8 @@ class ExtensionOperatorTest(fixtures.TestBase, testing.AssertsCompiledSQL):
 
     def test_contains(self):
         class MyType(UserDefinedType):
+            cache_ok = True
+
             class comparator_factory(UserDefinedType.Comparator):
                 def contains(self, other, **kw):
                     return self.op("->")(other)
@@ -664,6 +666,8 @@ class ExtensionOperatorTest(fixtures.TestBase, testing.AssertsCompiledSQL):
 
     def test_getitem(self):
         class MyType(UserDefinedType):
+            cache_ok = True
+
             class comparator_factory(UserDefinedType.Comparator):
                 def __getitem__(self, index):
                     return self.op("->")(index)
@@ -682,6 +686,8 @@ class ExtensionOperatorTest(fixtures.TestBase, testing.AssertsCompiledSQL):
 
     def test_lshift(self):
         class MyType(UserDefinedType):
+            cache_ok = True
+
             class comparator_factory(UserDefinedType.Comparator):
                 def __lshift__(self, other):
                     return self.op("->")(other)
@@ -690,6 +696,8 @@ class ExtensionOperatorTest(fixtures.TestBase, testing.AssertsCompiledSQL):
 
     def test_rshift(self):
         class MyType(UserDefinedType):
+            cache_ok = True
+
             class comparator_factory(UserDefinedType.Comparator):
                 def __rshift__(self, other):
                     return self.op("->")(other)