diff --git a/planner/core/expression_rewriter.go b/planner/core/expression_rewriter.go
index d6b6a680498cf..a241b1133a27a 100644
--- a/planner/core/expression_rewriter.go
+++ b/planner/core/expression_rewriter.go
@@ -42,13 +42,11 @@ func evalAstExpr(ctx sessionctx.Context, expr ast.ExprNode) (types.Datum, error)
 	if val, ok := expr.(*driver.ValueExpr); ok {
 		return val.Datum, nil
 	}
-	b := &PlanBuilder{
-		ctx:       ctx,
-		colMapper: make(map[*ast.ColumnNameExpr]int),
-	}
+	var is infoschema.InfoSchema
 	if ctx.GetSessionVars().TxnCtx.InfoSchema != nil {
-		b.is = ctx.GetSessionVars().TxnCtx.InfoSchema.(infoschema.InfoSchema)
+		is = ctx.GetSessionVars().TxnCtx.InfoSchema.(infoschema.InfoSchema)
 	}
+	b := NewPlanBuilder(ctx, is)
 	fakePlan := LogicalTableDual{}.Init(ctx)
 	newExpr, _, err := b.rewrite(expr, fakePlan, nil, true)
 	if err != nil {
diff --git a/planner/core/indexmerge_test.go b/planner/core/indexmerge_test.go
index 2ff5f09c91ef0..07ed20fb2ac69 100644
--- a/planner/core/indexmerge_test.go
+++ b/planner/core/indexmerge_test.go
@@ -16,7 +16,6 @@ package core
 import (
 	. "github.com/pingcap/check"
 	"github.com/pingcap/parser"
-	"github.com/pingcap/parser/ast"
 	"github.com/pingcap/parser/model"
 	"github.com/pingcap/tidb/infoschema"
 	"github.com/pingcap/tidb/sessionctx"
@@ -108,11 +107,7 @@ func (s *testIndexMergeSuite) TestIndexMergePathGenerateion(c *C) {
 		stmt, err := s.ParseOneStmt(tc.sql, "", "")
 		c.Assert(err, IsNil, comment)
 		Preprocess(s.ctx, stmt, s.is)
-		builder := &PlanBuilder{
-			ctx:       MockContext(),
-			is:        s.is,
-			colMapper: make(map[*ast.ColumnNameExpr]int),
-		}
+		builder := NewPlanBuilder(MockContext(), s.is)
 		p, err := builder.Build(stmt)
 		if err != nil {
 			c.Assert(err.Error(), Equals, tc.idxMergeDigest, comment)
diff --git a/planner/core/logical_plan_builder.go b/planner/core/logical_plan_builder.go
index fd502d0b5ad20..d22ee24afa1c0 100644
--- a/planner/core/logical_plan_builder.go
+++ b/planner/core/logical_plan_builder.go
@@ -2293,7 +2293,7 @@ func (b *PlanBuilder) buildDataSource(tn *ast.TableName) (LogicalPlan, error) {
 	if err != nil {
 		return nil, err
 	}
-	if txn.Valid() && !txn.IsReadOnly() {
+	if txn.Valid() && !txn.IsReadOnly() && !isMemDB {
 		us := LogicalUnionScan{}.Init(b.ctx)
 		us.SetChildren(ds)
 		result = us
diff --git a/planner/core/logical_plan_test.go b/planner/core/logical_plan_test.go
index 25d94c1cf516b..5c1586e2548e9 100644
--- a/planner/core/logical_plan_test.go
+++ b/planner/core/logical_plan_test.go
@@ -21,7 +21,6 @@ import (
 
 	. "github.com/pingcap/check"
 	"github.com/pingcap/parser"
-	"github.com/pingcap/parser/ast"
 	"github.com/pingcap/parser/model"
 	"github.com/pingcap/parser/mysql"
 	"github.com/pingcap/parser/terror"
@@ -1714,11 +1713,7 @@ func (s *testPlanSuite) TestVisitInfo(c *C) {
 		stmt, err := s.ParseOneStmt(tt.sql, "", "")
 		c.Assert(err, IsNil, comment)
 		Preprocess(s.ctx, stmt, s.is)
-		builder := &PlanBuilder{
-			colMapper: make(map[*ast.ColumnNameExpr]int),
-			ctx:       MockContext(),
-			is:        s.is,
-		}
+		builder := NewPlanBuilder(MockContext(), s.is)
 		builder.ctx.GetSessionVars().HashJoinConcurrency = 1
 		_, err = builder.Build(stmt)
 		c.Assert(err, IsNil, comment)
@@ -1832,11 +1827,7 @@ func (s *testPlanSuite) TestUnion(c *C) {
 		stmt, err := s.ParseOneStmt(tt.sql, "", "")
 		c.Assert(err, IsNil, comment)
 		Preprocess(s.ctx, stmt, s.is)
-		builder := &PlanBuilder{
-			ctx:       MockContext(),
-			is:        s.is,
-			colMapper: make(map[*ast.ColumnNameExpr]int),
-		}
+		builder := NewPlanBuilder(MockContext(), s.is)
 		plan, err := builder.Build(stmt)
 		if tt.err {
 			c.Assert(err, NotNil)
@@ -1964,11 +1955,7 @@ func (s *testPlanSuite) TestTopNPushDown(c *C) {
 		stmt, err := s.ParseOneStmt(tt.sql, "", "")
 		c.Assert(err, IsNil, comment)
 		Preprocess(s.ctx, stmt, s.is)
-		builder := &PlanBuilder{
-			ctx:       MockContext(),
-			is:        s.is,
-			colMapper: make(map[*ast.ColumnNameExpr]int),
-		}
+		builder := NewPlanBuilder(MockContext(), s.is)
 		p, err := builder.Build(stmt)
 		c.Assert(err, IsNil)
 		p, err = logicalOptimize(builder.optFlag, p.(LogicalPlan))
@@ -2075,11 +2062,7 @@ func (s *testPlanSuite) TestOuterJoinEliminator(c *C) {
 		stmt, err := s.ParseOneStmt(tt.sql, "", "")
 		c.Assert(err, IsNil, comment)
 		Preprocess(s.ctx, stmt, s.is)
-		builder := &PlanBuilder{
-			ctx:       MockContext(),
-			is:        s.is,
-			colMapper: make(map[*ast.ColumnNameExpr]int),
-		}
+		builder := NewPlanBuilder(MockContext(), s.is)
 		p, err := builder.Build(stmt)
 		c.Assert(err, IsNil)
 		p, err = logicalOptimize(builder.optFlag, p.(LogicalPlan))
@@ -2106,11 +2089,7 @@ func (s *testPlanSuite) TestSelectView(c *C) {
 		stmt, err := s.ParseOneStmt(tt.sql, "", "")
 		c.Assert(err, IsNil, comment)
 		Preprocess(s.ctx, stmt, s.is)
-		builder := &PlanBuilder{
-			ctx:       MockContext(),
-			is:        s.is,
-			colMapper: make(map[*ast.ColumnNameExpr]int),
-		}
+		builder := NewPlanBuilder(MockContext(), s.is)
 		p, err := builder.Build(stmt)
 		c.Assert(err, IsNil)
 		p, err = logicalOptimize(builder.optFlag, p.(LogicalPlan))
@@ -2333,11 +2312,7 @@ func (s *testPlanSuite) TestWindowFunction(c *C) {
 		stmt, err := s.ParseOneStmt(tt.sql, "", "")
 		c.Assert(err, IsNil, comment)
 		Preprocess(s.ctx, stmt, s.is)
-		builder := &PlanBuilder{
-			ctx:       MockContext(),
-			is:        s.is,
-			colMapper: make(map[*ast.ColumnNameExpr]int),
-		}
+		builder := NewPlanBuilder(MockContext(), s.is)
 		p, err := builder.Build(stmt)
 		if err != nil {
 			c.Assert(err.Error(), Equals, tt.result, comment)
@@ -2418,11 +2393,7 @@ func (s *testPlanSuite) TestSkylinePruning(c *C) {
 		stmt, err := s.ParseOneStmt(tt.sql, "", "")
 		c.Assert(err, IsNil, comment)
 		Preprocess(s.ctx, stmt, s.is)
-		builder := &PlanBuilder{
-			ctx:       MockContext(),
-			is:        s.is,
-			colMapper: make(map[*ast.ColumnNameExpr]int),
-		}
+		builder := NewPlanBuilder(MockContext(), s.is)
 		p, err := builder.Build(stmt)
 		if err != nil {
 			c.Assert(err.Error(), Equals, tt.result, comment)
diff --git a/planner/core/optimizer.go b/planner/core/optimizer.go
index d41dadde11710..ab7d09117fe19 100644
--- a/planner/core/optimizer.go
+++ b/planner/core/optimizer.go
@@ -74,11 +74,7 @@ type logicalOptRule interface {
 func BuildLogicalPlan(ctx sessionctx.Context, node ast.Node, is infoschema.InfoSchema) (Plan, error) {
 	ctx.GetSessionVars().PlanID = 0
 	ctx.GetSessionVars().PlanColumnID = 0
-	builder := &PlanBuilder{
-		ctx:       ctx,
-		is:        is,
-		colMapper: make(map[*ast.ColumnNameExpr]int),
-	}
+	builder := NewPlanBuilder(ctx, is)
 	p, err := builder.Build(node)
 	if err != nil {
 		return nil, err
diff --git a/planner/core/planbuilder_test.go b/planner/core/planbuilder_test.go
index 3c1487d1a3637..282e95a2cd9ff 100644
--- a/planner/core/planbuilder_test.go
+++ b/planner/core/planbuilder_test.go
@@ -94,9 +94,7 @@ func (s *testPlanBuilderSuite) TestGetPathByIndexName(c *C) {
 }
 
 func (s *testPlanBuilderSuite) TestRewriterPool(c *C) {
-	builder := &PlanBuilder{
-		ctx: MockContext(),
-	}
+	builder := NewPlanBuilder(MockContext(), nil)
 
 	// Make sure PlanBuilder.getExpressionRewriter() provides clean rewriter from pool.
 	// First, pick one rewriter from the pool and make it dirty.
@@ -149,7 +147,7 @@ func (s *testPlanBuilderSuite) TestDisableFold(c *C) {
 		stmt := st.(*ast.SelectStmt)
 		expr := stmt.Fields.Fields[0].Expr
 
-		builder := &PlanBuilder{ctx: ctx}
+		builder := NewPlanBuilder(ctx, nil)
 		builder.rewriterCounter++
 		rewriter := builder.getExpressionRewriter(nil)
 		c.Assert(rewriter, NotNil)
diff --git a/planner/core/rule_partition_processor.go b/planner/core/rule_partition_processor.go
index 4b04412e5c02a..f2c3ebc59db63 100644
--- a/planner/core/rule_partition_processor.go
+++ b/planner/core/rule_partition_processor.go
@@ -45,12 +45,11 @@ func (s *partitionProcessor) optimize(lp LogicalPlan) (LogicalPlan, error) {
 
 func (s *partitionProcessor) rewriteDataSource(lp LogicalPlan) (LogicalPlan, error) {
 	// Assert there will not be sel -> sel in the ast.
-	switch lp.(type) {
+	switch p := lp.(type) {
 	case *DataSource:
-		return s.prune(lp.(*DataSource))
+		return s.prune(p)
 	case *LogicalUnionScan:
-		us := lp.(*LogicalUnionScan)
-		ds := us.Children()[0]
+		ds := p.Children()[0]
 		ds, err := s.prune(ds.(*DataSource))
 		if err != nil {
 			return nil, err
@@ -60,17 +59,16 @@ func (s *partitionProcessor) rewriteDataSource(lp LogicalPlan) (LogicalPlan, err
 			// Union->(UnionScan->DataSource1), (UnionScan->DataSource2)
 			children := make([]LogicalPlan, 0, len(ua.Children()))
 			for _, child := range ua.Children() {
-				usChild := LogicalUnionScan{}.Init(ua.ctx)
-				usChild.conditions = us.conditions
-				usChild.SetChildren(child)
-				children = append(children, usChild)
+				us := LogicalUnionScan{conditions: p.conditions}.Init(ua.ctx)
+				us.SetChildren(child)
+				children = append(children, us)
 			}
 			ua.SetChildren(children...)
 			return ua, nil
 		}
 		// Only one partition, no union all.
-		us.SetChildren(ds)
-		return us, nil
+		p.SetChildren(ds)
+		return p, nil
 	default:
 		children := lp.Children()
 		for i, child := range children {