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

1 files changed, 6 insertions, 8 deletions

diff --git a/lib/sqlalchemy/sql/elements.py b/lib/sqlalchemy/sql/elements.py
index af5eab257..6ce505412 100644
--- a/lib/sqlalchemy/sql/elements.py
+++ b/lib/sqlalchemy/sql/elements.py
@@ -215,6 +215,7 @@ class ClauseElement(
     _is_select_statement = False
     _is_bind_parameter = False
     _is_clause_list = False
+    _is_lambda_element = False
 
     _order_by_label_element = None
 
@@ -1337,9 +1338,6 @@ class BindParameter(roles.InElementRole, ColumnElement):
 
                 :ref:`change_4808`.
 
-
-
-
         """
 
         if required is NO_ARG:
@@ -1406,15 +1404,15 @@ class BindParameter(roles.InElementRole, ColumnElement):
         the context of an expanding IN against a tuple.
 
         """
-        cloned = self._clone()
+        cloned = self._clone(maintain_key=True)
         cloned._expanding_in_types = types
         return cloned
 
-    def _with_value(self, value):
+    def _with_value(self, value, maintain_key=False):
         """Return a copy of this :class:`.BindParameter` with the given value
         set.
         """
-        cloned = self._clone()
+        cloned = self._clone(maintain_key=maintain_key)
         cloned.value = value
         cloned.callable = None
         cloned.required = False
@@ -1442,9 +1440,9 @@ class BindParameter(roles.InElementRole, ColumnElement):
         c.type = type_
         return c
 
-    def _clone(self):
+    def _clone(self, maintain_key=False):
         c = ClauseElement._clone(self)
-        if self.unique:
+        if not maintain_key and self.unique:
             c.key = _anonymous_label(
                 "%%(%d %s)s" % (id(c), c._orig_key or "param")
             )