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, 17 insertions, 4 deletions

diff --git a/lib/sqlalchemy/engine/default.py b/lib/sqlalchemy/engine/default.py
index 9574e9980..e91e34f00 100644
--- a/lib/sqlalchemy/engine/default.py
+++ b/lib/sqlalchemy/engine/default.py
@@ -321,10 +321,23 @@ class DefaultDialect(interfaces.Dialect):
 
     @util.memoized_property
     def _supports_statement_cache(self):
-        return (
-            self.__class__.__dict__.get("supports_statement_cache", False)
-            is True
-        )
+        ssc = self.__class__.__dict__.get("supports_statement_cache", None)
+        if ssc is None:
+            util.warn(
+                "Dialect %s:%s will not make use of SQL compilation caching "
+                "as it does not set the 'supports_statement_cache' attribute "
+                "to ``True``.  This can have "
+                "significant performance implications including some "
+                "performance degradations in comparison to prior SQLAlchemy "
+                "versions.  Dialect maintainers should seek to set this "
+                "attribute to True after appropriate development and testing "
+                "for SQLAlchemy 1.4 caching support.   Alternatively, this "
+                "attribute may be set to False which will disable this "
+                "warning." % (self.name, self.driver),
+                code="cprf",
+            )
+
+        return bool(ssc)
 
     @util.memoized_property
     def _type_memos(self):