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, 6 insertions, 1 deletions

diff --git a/lib/sqlalchemy/sql/elements.py b/lib/sqlalchemy/sql/elements.py
index a7b86d3ec..00270c9b5 100644
--- a/lib/sqlalchemy/sql/elements.py
+++ b/lib/sqlalchemy/sql/elements.py
@@ -3646,6 +3646,8 @@ class CollectionAggregate(UnaryExpression):
 
     """
 
+    inherit_cache = True
+
     @classmethod
     def _create_any(cls, expr):
         """Produce an ANY expression.
@@ -3953,7 +3955,7 @@ class IndexExpression(BinaryExpression):
     """Represent the class of expressions that are like an "index"
     operation."""
 
-    pass
+    inherit_cache = True
 
 
 class GroupedElement(ClauseElement):
@@ -5040,14 +5042,17 @@ class _IdentifiedClause(Executable, ClauseElement):
 
 class SavepointClause(_IdentifiedClause):
     __visit_name__ = "savepoint"
+    inherit_cache = False
 
 
 class RollbackToSavepointClause(_IdentifiedClause):
     __visit_name__ = "rollback_to_savepoint"
+    inherit_cache = False
 
 
 class ReleaseSavepointClause(_IdentifiedClause):
     __visit_name__ = "release_savepoint"
+    inherit_cache = False
 
 
 class quoted_name(util.MemoizedSlots, str):