From f3b6f4f8da5223fae0a1dd948d4266b2e49e317c Mon Sep 17 00:00:00 2001 From: Mike Bayer Date: Tue, 14 Mar 2017 12:00:56 -0400 Subject: 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 --- lib/sqlalchemy/sql/compiler.py | 24 ++++++++++++++++++++++++ 1 file changed, 24 insertions(+) (limited to 'lib/sqlalchemy/sql/compiler.py') 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): -- cgit v1.2.1