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, 4 insertions, 0 deletions

diff --git a/lib/sqlalchemy/sql/functions.py b/lib/sqlalchemy/sql/functions.py
index fff2defe0..5d2e78065 100644
--- a/lib/sqlalchemy/sql/functions.py
+++ b/lib/sqlalchemy/sql/functions.py
@@ -917,6 +917,7 @@ class GenericFunction(Function, metaclass=_GenericMeta):
 
         class as_utc(GenericFunction):
             type = DateTime
+            inherit_cache = True
 
         print(select(func.as_utc()))
 
@@ -931,6 +932,7 @@ class GenericFunction(Function, metaclass=_GenericMeta):
         class as_utc(GenericFunction):
             type = DateTime
             package = "time"
+            inherit_cache = True
 
     The above function would be available from :data:`.func`
     using the package name ``time``::
@@ -948,6 +950,7 @@ class GenericFunction(Function, metaclass=_GenericMeta):
             package = "geo"
             name = "ST_Buffer"
             identifier = "buffer"
+            inherit_cache = True
 
     The above function will render as follows::
 
@@ -966,6 +969,7 @@ class GenericFunction(Function, metaclass=_GenericMeta):
             package = "geo"
             name = quoted_name("ST_Buffer", True)
             identifier = "buffer"
+            inherit_cache = True
 
     The above function will render as::