introduce deferred lambdas

The coercions system allows us to add in lambdas as arguments
to Core and ORM elements without changing them at all.   By allowing
the lambda to produce a deterministic cache key where we can also
cheat and yank out literal parameters means we can move towards
having 90% of "baked" functionality in a clearer way right in
Core / ORM.

As a second step, we can have whole statements inside the lambda,
and can then add generation with __add__(), so then we have
100% of "baked" functionality with full support of ad-hoc
literal values.

Adds some more short_selects tests for the moment for comparison.

Other tweaks inside cache key generation as we're trying to
approach a certain level of performance such that we can
remove the use of "baked" from the loader strategies.

As we have not yet closed #4639, however the caching feature
has been fully integrated as of
b0cfa7379cf8513a821a3dbe3028c4965d9f85bd, we will also
add complete caching documentation here and close that issue
as well.

Closes: #4639
Fixes: #5380
Change-Id: If91f61527236fd4d7ae3cad1f24c38be921c90ba

4 files changed, 8 insertions, 5 deletions

diff --git a/lib/sqlalchemy/engine/base.py b/lib/sqlalchemy/engine/base.py
index ed4133dbb..9ac61fe12 100644
--- a/lib/sqlalchemy/engine/base.py
+++ b/lib/sqlalchemy/engine/base.py
@@ -1046,6 +1046,7 @@ class Connection(Connectable):
                 distilled_parameters,
                 _EMPTY_EXECUTION_OPTS,
             )
+
         try:
             meth = object_._execute_on_connection
         except AttributeError as err:
diff --git a/lib/sqlalchemy/engine/create.py b/lib/sqlalchemy/engine/create.py
index cc138412b..c199c21e0 100644
--- a/lib/sqlalchemy/engine/create.py
+++ b/lib/sqlalchemy/engine/create.py
@@ -459,8 +459,7 @@ def create_engine(url, **kwargs):
 
      .. seealso::
 
-        ``engine_caching`` - TODO: this will be an upcoming section describing
-        the SQL caching system.
+        :ref:`sql_caching`
 
      .. versionadded:: 1.4
 
diff --git a/lib/sqlalchemy/engine/default.py b/lib/sqlalchemy/engine/default.py
index 51bff223c..f1fc505ac 100644
--- a/lib/sqlalchemy/engine/default.py
+++ b/lib/sqlalchemy/engine/default.py
@@ -1031,7 +1031,7 @@ class DefaultExecutionContext(interfaces.ExecutionContext):
         if self.compiled.cache_key is None:
             return "no key %.5fs" % (now - self.compiled._gen_time,)
         elif self.cache_hit:
-            return "cached for %.4gs" % (now - self.compiled._gen_time,)
+            return "cached since %.4gs ago" % (now - self.compiled._gen_time,)
         else:
             return "generated in %.5fs" % (now - self.compiled._gen_time,)
 
diff --git a/lib/sqlalchemy/engine/result.py b/lib/sqlalchemy/engine/result.py
index f75cba57d..7df17cf22 100644
--- a/lib/sqlalchemy/engine/result.py
+++ b/lib/sqlalchemy/engine/result.py
@@ -963,7 +963,10 @@ class Result(InPlaceGenerative):
             else:
                 return None
 
-        make_row = self._row_getter
+        if scalar and self._source_supports_scalars:
+            make_row = None
+        else:
+            make_row = self._row_getter
 
         row = make_row(row) if make_row else row
 
@@ -1016,7 +1019,7 @@ class Result(InPlaceGenerative):
             if post_creational_filter:
                 row = post_creational_filter(row)
 
-        if scalar and row:
+        if scalar and make_row:
             return row[0]
         else:
             return row