Add "empty in" strategies; default to "static"

The longstanding behavior of the :meth:`.Operators.in_` and
:meth:`.Operators.not_in_` operators emitting a warning when
the right-hand condition is an empty sequence has been revised;
a new flag :paramref:`.create_engine.empty_in_strategy` allows an
empty "IN" expression to generate a simple boolean expression, or
to invoke the previous behavior of dis-equating the expression to
itself, with or without a warning.  The default behavior is now
to emit the simple boolean expression, allowing an empty IN to
be evaulated without any performance penalty.

Change-Id: I65cc37f2d7cf65a59bf217136c42fee446929352
Fixes: #3907

1 files changed, 7 insertions, 16 deletions

diff --git a/lib/sqlalchemy/sql/default_comparator.py b/lib/sqlalchemy/sql/default_comparator.py
index 7498bbe5d..d409ebacc 100644
--- a/lib/sqlalchemy/sql/default_comparator.py
+++ b/lib/sqlalchemy/sql/default_comparator.py
@@ -146,23 +146,14 @@ def _in_impl(expr, op, seq_or_selectable, negate_op, **kw):
         else:
             o = expr._bind_param(op, o)
         args.append(o)
-    if len(args) == 0:
 
-        # Special case handling for empty IN's, behave like
-        # comparison against zero row selectable.  We use != to
-        # build the contradiction as it handles NULL values
-        # appropriately, i.e. "not (x IN ())" should not return NULL
-        # values for x.
-
-        util.warn('The IN-predicate on "%s" was invoked with an '
-                  'empty sequence. This results in a '
-                  'contradiction, which nonetheless can be '
-                  'expensive to evaluate.  Consider alternative '
-                  'strategies for improved performance.' % expr)
-        if op is operators.in_op:
-            return expr != expr
-        else:
-            return expr == expr
+    if len(args) == 0:
+        op, negate_op = (
+            operators.empty_in_op,
+            operators.empty_notin_op) if op is operators.in_op \
+            else (
+                operators.empty_notin_op,
+                operators.empty_in_op)
 
     return _boolean_compare(expr, op,
                             ClauseList(*args).self_group(against=op),