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/lib/sqlalchemy/dialects/postgresql/hstore.py b/lib/sqlalchemy/dialects/postgresql/hstore.py
index 2ade4b7c1..77220a33a 100644
--- a/lib/sqlalchemy/dialects/postgresql/hstore.py
+++ b/lib/sqlalchemy/dialects/postgresql/hstore.py
@@ -273,41 +273,49 @@ class hstore(sqlfunc.GenericFunction):
 
     type = HSTORE
     name = "hstore"
+    inherit_cache = True
 
 
 class _HStoreDefinedFunction(sqlfunc.GenericFunction):
     type = sqltypes.Boolean
     name = "defined"
+    inherit_cache = True
 
 
 class _HStoreDeleteFunction(sqlfunc.GenericFunction):
     type = HSTORE
     name = "delete"
+    inherit_cache = True
 
 
 class _HStoreSliceFunction(sqlfunc.GenericFunction):
     type = HSTORE
     name = "slice"
+    inherit_cache = True
 
 
 class _HStoreKeysFunction(sqlfunc.GenericFunction):
     type = ARRAY(sqltypes.Text)
     name = "akeys"
+    inherit_cache = True
 
 
 class _HStoreValsFunction(sqlfunc.GenericFunction):
     type = ARRAY(sqltypes.Text)
     name = "avals"
+    inherit_cache = True
 
 
 class _HStoreArrayFunction(sqlfunc.GenericFunction):
     type = ARRAY(sqltypes.Text)
     name = "hstore_to_array"
+    inherit_cache = True
 
 
 class _HStoreMatrixFunction(sqlfunc.GenericFunction):
     type = ARRAY(sqltypes.Text)
     name = "hstore_to_matrix"
+    inherit_cache = True
 
 
 #