expression: support ConstItem() for expression (#10004)

expression/column.go

@@ -123,6 +123,11 @@ func (col *CorrelatedColumn) IsCorrelated() bool {
 	return true
 }
 
+// ConstItem implements Expression interface.
+func (col *CorrelatedColumn) ConstItem() bool {
+	return false
+}
+
 // Decorrelate implements Expression interface.
 func (col *CorrelatedColumn) Decorrelate(schema *Schema) Expression {
 	if !schema.Contains(&col.Column) {
@@ -297,6 +302,11 @@ func (col *Column) IsCorrelated() bool {
 	return false
 }
 
+// ConstItem implements Expression interface.
+func (col *Column) ConstItem() bool {
+	return false
+}
+
 // Decorrelate implements Expression interface.
 func (col *Column) Decorrelate(_ *Schema) Expression {
 	return col

expression/column_test.go

@@ -43,6 +43,7 @@ func (s *testEvaluatorSuite) TestColumn(c *C) {
 	c.Assert(corCol.Equal(nil, corCol), IsTrue)
 	c.Assert(corCol.Equal(nil, invalidCorCol), IsFalse)
 	c.Assert(corCol.IsCorrelated(), IsTrue)
+	c.Assert(corCol.ConstItem(), IsFalse)
 	c.Assert(corCol.Decorrelate(schema).Equal(nil, col), IsTrue)
 	c.Assert(invalidCorCol.Decorrelate(schema).Equal(nil, invalidCorCol), IsTrue)
 

expression/constant.go

@@ -311,6 +311,11 @@ func (c *Constant) IsCorrelated() bool {
 	return false
 }
 
+// ConstItem implements Expression interface.
+func (c *Constant) ConstItem() bool {
+	return true
+}
+
 // Decorrelate implements Expression interface.
 func (c *Constant) Decorrelate(_ *Schema) Expression {
 	return c

expression/expression.go

@@ -79,6 +79,14 @@ type Expression interface {
 	// IsCorrelated checks if this expression has correlated key.
 	IsCorrelated() bool
 
+	// ConstItem checks if this expression is constant item, regardless of query evaluation state.
+	// An expression is constant item if it:
+	// refers no tables.
+	// refers no subqueries that refers any tables.
+	// refers no non-deterministic functions.
+	// refers no statement parameters.
+	ConstItem() bool
+
 	// Decorrelate try to decorrelate the expression by schema.
 	Decorrelate(schema *Schema) Expression
 

expression/expression_test.go

@@ -61,6 +61,7 @@ func (s *testEvaluatorSuite) TestConstant(c *C) {
 
 	sc := &stmtctx.StatementContext{TimeZone: time.Local}
 	c.Assert(Zero.IsCorrelated(), IsFalse)
+	c.Assert(Zero.ConstItem(), IsTrue)
 	c.Assert(Zero.Decorrelate(nil).Equal(s.ctx, Zero), IsTrue)
 	c.Assert(Zero.HashCode(sc), DeepEquals, []byte{0x0, 0x8, 0x0})
 	c.Assert(Zero.Equal(s.ctx, One), IsFalse)
@@ -85,6 +86,19 @@ func (s *testEvaluatorSuite) TestIsBinaryLiteral(c *C) {
 	c.Assert(IsBinaryLiteral(con), IsFalse)
 }
 
+func (s *testEvaluatorSuite) TestConstItem(c *C) {
+	defer testleak.AfterTest(c)()
+
+	sf := newFunction(ast.Rand)
+	c.Assert(sf.ConstItem(), Equals, false)
+	sf = newFunction(ast.UUID)
+	c.Assert(sf.ConstItem(), Equals, false)
+	sf = newFunction(ast.GetParam, One)
+	c.Assert(sf.ConstItem(), Equals, false)
+	sf = newFunction(ast.Abs, One)
+	c.Assert(sf.ConstItem(), Equals, true)
+}
+
 type testTableBuilder struct {
 	tableName   string
 	columnNames []string

expression/scalar_function.go

@@ -165,6 +165,20 @@ func (sf *ScalarFunction) IsCorrelated() bool {
 	return false
 }
 
+// ConstItem implements Expression interface.
+func (sf *ScalarFunction) ConstItem() bool {
+	// Note: some unfoldable functions are deterministic, we use unFoldableFunctions here for simplification.
+	if _, ok := unFoldableFunctions[sf.FuncName.L]; ok {
+		return false
+	}
+	for _, arg := range sf.GetArgs() {
+		if !arg.ConstItem() {
+			return false
+		}
+	}
+	return true
+}
+
 // Decorrelate implements Expression interface.
 func (sf *ScalarFunction) Decorrelate(schema *Schema) Expression {
 	for i, arg := range sf.GetArgs() {

expression/scalar_function_test.go

@@ -40,6 +40,7 @@ func (s *testEvaluatorSuite) TestScalarFunction(c *C) {
 	c.Assert(err, IsNil)
 	c.Assert(res, DeepEquals, []byte{0x22, 0x6c, 0x74, 0x28, 0x66, 0x65, 0x69, 0x2e, 0x68, 0x61, 0x6e, 0x2c, 0x20, 0x31, 0x29, 0x22})
 	c.Assert(sf.IsCorrelated(), IsFalse)
+	c.Assert(sf.ConstItem(), IsFalse)
 	c.Assert(sf.Decorrelate(nil).Equal(s.ctx, sf), IsTrue)
 	c.Assert(sf.HashCode(sc), DeepEquals, []byte{0x3, 0x4, 0x6c, 0x74, 0x1, 0x80, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x1, 0x0, 0x5, 0xbf, 0xf0, 0x0, 0x0, 0x0, 0x0, 0x0, 0x0})