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, 9 insertions, 0 deletions

diff --git a/lib/sqlalchemy/testing/assertions.py b/lib/sqlalchemy/testing/assertions.py
index 234ab4b93..2acf15195 100644
--- a/lib/sqlalchemy/testing/assertions.py
+++ b/lib/sqlalchemy/testing/assertions.py
@@ -546,6 +546,15 @@ class AssertsCompiledSQL:
         # are the "self.statement" element
         c = CheckCompilerAccess(clause).compile(dialect=dialect, **kw)
 
+        if isinstance(clause, sqltypes.TypeEngine):
+            cache_key_no_warnings = clause._static_cache_key
+            if cache_key_no_warnings:
+                hash(cache_key_no_warnings)
+        else:
+            cache_key_no_warnings = clause._generate_cache_key()
+            if cache_key_no_warnings:
+                hash(cache_key_no_warnings[0])
+
         param_str = repr(getattr(c, "params", {}))
         param_str = param_str.encode("utf-8").decode("ascii", "ignore")
         print(("\nSQL String:\n" + str(c) + param_str).encode("utf-8"))