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

3 files changed, 62 insertions, 17 deletions

diff --git a/lib/sqlalchemy/sql/compiler.py b/lib/sqlalchemy/sql/compiler.py
index bfa22c206..a1d5a879d 100644
--- a/lib/sqlalchemy/sql/compiler.py
+++ b/lib/sqlalchemy/sql/compiler.py
@@ -1003,6 +1003,30 @@ class SQLCompiler(Compiled):
         return "NOT %s" % self.visit_binary(
             binary, override_operator=operators.match_op)
 
+    def _emit_empty_in_warning(self):
+        util.warn(
+            'The IN-predicate was invoked with an '
+            'empty sequence. This results in a '
+            'contradiction, which nonetheless can be '
+            'expensive to evaluate.  Consider alternative '
+            'strategies for improved performance.')
+
+    def visit_empty_in_op_binary(self, binary, operator, **kw):
+        if self.dialect._use_static_in:
+            return "1 != 1"
+        else:
+            if self.dialect._warn_on_empty_in:
+                self._emit_empty_in_warning()
+            return self.process(binary.left != binary.left)
+
+    def visit_empty_notin_op_binary(self, binary, operator, **kw):
+        if self.dialect._use_static_in:
+            return "1 = 1"
+        else:
+            if self.dialect._warn_on_empty_in:
+                self._emit_empty_in_warning()
+            return self.process(binary.left == binary.left)
+
     def visit_binary(self, binary, override_operator=None,
                      eager_grouping=False, **kw):
 
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),
diff --git a/lib/sqlalchemy/sql/operators.py b/lib/sqlalchemy/sql/operators.py
index 1690d546b..01bee62cf 100644
--- a/lib/sqlalchemy/sql/operators.py
+++ b/lib/sqlalchemy/sql/operators.py
@@ -458,6 +458,17 @@ class ColumnOperators(Operators):
         "other" may be a tuple/list of column expressions,
         or a :func:`~.expression.select` construct.
 
+        In the case that ``other`` is an empty sequence, the compiler
+        produces an "empty in" expression.   This defaults to the
+        expression "1 != 1" to produce false in all cases.  The
+        :paramref:`.create_engine.empty_in_strategy` may be used to
+        alter this behavior.
+
+        .. versionchanged:: 1.2  The :meth:`.ColumnOperators.in_` and
+           :meth:`.ColumnOperators.notin_` operators
+           now produce a "static" expression for an empty IN sequence
+           by default.
+
         """
         return self.operate(in_op, other)
 
@@ -467,7 +478,16 @@ class ColumnOperators(Operators):
         This is equivalent to using negation with
         :meth:`.ColumnOperators.in_`, i.e. ``~x.in_(y)``.
 
-        .. versionadded:: 0.8
+        In the case that ``other`` is an empty sequence, the compiler
+        produces an "empty not in" expression.   This defaults to the
+        expression "1 = 1" to produce true in all cases.  The
+        :paramref:`.create_engine.empty_in_strategy` may be used to
+        alter this behavior.
+
+        .. versionchanged:: 1.2  The :meth:`.ColumnOperators.in_` and
+           :meth:`.ColumnOperators.notin_` operators
+           now produce a "static" expression for an empty IN sequence
+           by default.
 
         .. seealso::
 
@@ -957,6 +977,14 @@ def comma_op(a, b):
     raise NotImplementedError()
 
 
+def empty_in_op(a, b):
+    raise NotImplementedError()
+
+
+def empty_notin_op(a, b):
+    raise NotImplementedError()
+
+
 def concat_op(a, b):
     return a.concat(b)
 
@@ -1073,6 +1101,8 @@ _PRECEDENCE = {
     ne: 5,
     is_distinct_from: 5,
     isnot_distinct_from: 5,
+    empty_in_op: 5,
+    empty_notin_op: 5,
     gt: 5,
     lt: 5,
     ge: 5,