planner: add memo_expr to represent the unified logical op and group expression

pkg/planner/cascades/memo/BUILD.bazel

@@ -7,6 +7,7 @@ go_library(
         "group_expr.go",
         "group_id_generator.go",
         "memo.go",
+        "memo_expr.go",
     ],
     importpath = "github.com/pingcap/tidb/pkg/planner/cascades/memo",
     visibility = ["//visibility:public"],

pkg/planner/cascades/memo/memo_expr.go

@@ -0,0 +1,74 @@
+// Copyright 2024 PingCAP, Inc.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package memo
+
+import (
+	"github.com/pingcap/tidb/pkg/planner/core/base"
+)
+
+// MemoExpression is a unified representation of logical and group expression.
+// 1: when stepping into memo optimization, we may convert a logical plan tree into a memoExpr,
+//
+//	so each node of them is simply an LP.
+//
+// 2: when XForm is called from a specific rule, substitution happened, the output may be a mixture
+//
+//	of logical plan and group expression.
+//
+// # Thus, memoExpr is responsible for representing both of them, leveraging the unified portal of
+//
+// MemoExpression managing memo group generation and hashing functionality.
+//
+//revive:disable:exported
+type MemoExpression struct {
+	LP base.LogicalPlan
+
+	GE *GroupExpression
+
+	Inputs []*MemoExpression
+}
+
+// IsLogicalPlan checks whether this me is a logical plan.
+func (me *MemoExpression) IsLogicalPlan() bool {
+	return me.LP != nil && me.GE == nil
+}
+
+// IsGroupExpression checks whether this me is a group expression.
+func (me *MemoExpression) IsGroupExpression() bool {
+	return me.LP == nil && me.GE != nil
+}
+
+// NewMemoExpressionFromPlan init a memeExpression with a logical plan.
+func NewMemoExpressionFromPlan(plan base.LogicalPlan) *MemoExpression {
+	return &MemoExpression{
+		LP: plan,
+	}
+}
+
+// NewMemoExpressionFromGroupExpression inits a memoExpression with an existed group expression.
+func NewMemoExpressionFromGroupExpression(ge *GroupExpression) *MemoExpression {
+	return &MemoExpression{
+		GE: ge,
+	}
+}
+
+// NewMemoExpressionFromPlanAndInputs inits a memoExpression with mixed logical plan as current node,
+// and certain memoExpressions as its input.
+func NewMemoExpressionFromPlanAndInputs(plan base.LogicalPlan, inputs []*MemoExpression) *MemoExpression {
+	return &MemoExpression{
+		LP:     plan,
+		Inputs: inputs,
+	}
+}