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/lib/sqlalchemy/dialects/postgresql/ext.py b/lib/sqlalchemy/dialects/postgresql/ext.py
index f779a8010..e323da8be 100644
--- a/lib/sqlalchemy/dialects/postgresql/ext.py
+++ b/lib/sqlalchemy/dialects/postgresql/ext.py
@@ -54,6 +54,7 @@ class aggregate_order_by(expression.ColumnElement):
     __visit_name__ = "aggregate_order_by"
 
     stringify_dialect = "postgresql"
+    inherit_cache = False
 
     def __init__(self, target, *order_by):
         self.target = coercions.expect(roles.ExpressionElementRole, target)
@@ -99,6 +100,7 @@ class ExcludeConstraint(ColumnCollectionConstraint):
     __visit_name__ = "exclude_constraint"
 
     where = None
+    inherit_cache = False
 
     create_drop_stringify_dialect = "postgresql"