Use non-subquery form for empty IN

Revised the "EMPTY IN" expression to no longer rely upon using a subquery,
as this was causing some compatibility and performance problems. The new
approach for selected databases takes advantage of using a NULL-returning
IN expression combined with the usual "1 != 1" or "1 = 1" expression
appended by AND or OR. The expression is now the default for all backends
other than SQLite, which still had some compatibility issues regarding
tuple "IN" for older SQLite versions.

Third party dialects can still override how the "empty set" expression
renders by implementing a new compiler method
``def visit_empty_set_op_expr(self, type_, expand_op)``, which takes
precedence over the existing
``def visit_empty_set_expr(self, element_types)`` which remains in place.

Fixes: #6258
Fixes: #6397
Change-Id: I2df09eb00d2ad3b57039ae48128fdf94641b5e59

1 files changed, 10 insertions, 0 deletions

diff --git a/lib/sqlalchemy/sql/elements.py b/lib/sqlalchemy/sql/elements.py
index 696f3b249..e27b97802 100644
--- a/lib/sqlalchemy/sql/elements.py
+++ b/lib/sqlalchemy/sql/elements.py
@@ -1411,7 +1411,17 @@ class BindParameter(roles.InElementRole, ColumnElement):
         self.callable = callable_
         self.isoutparam = isoutparam
         self.required = required
+
+        # indicate an "expanding" parameter; the compiler sets this
+        # automatically in the compiler _render_in_expr_w_bindparam method
+        # for an IN expression
         self.expanding = expanding
+
+        # this is another hint to help w/ expanding and is typically
+        # set in the compiler _render_in_expr_w_bindparam method for an
+        # IN expression
+        self.expand_op = None
+
         self.literal_execute = literal_execute
         if _is_crud:
             self._is_crud = True