fewer schema allocations

Author: James Cor

sql/plan/tablealias.go

@@ -21,11 +21,12 @@ import (
 // TableAlias is a node that acts as a table with a given name.
 type TableAlias struct {
 	*UnaryNode
-	name    string
-	comment string
-	id      sql.TableId
-	cols    sql.ColSet
-	sch     sql.Schema
+	name      string
+	comment   string
+	id        sql.TableId
+	cols      sql.ColSet
+	sch       sql.Schema
+	cachedSch bool
 }
 
 var _ sql.RenameableNode = (*TableAlias)(nil)
@@ -34,13 +35,7 @@ var _ sql.CollationCoercible = (*TableAlias)(nil)
 
 // NewTableAlias returns a new Table alias node.
 func NewTableAlias(name string, node sql.Node) *TableAlias {
-	childSchema := node.Schema()
-	schema := make(sql.Schema, len(childSchema))
-	for i, col := range childSchema {
-		newCol := *col
-		newCol.Source = name
-		schema[i] = &newCol
-	}
+	schema := make(sql.Schema, len(node.Schema()))
 	ret := &TableAlias{
 		UnaryNode: &UnaryNode{Child: node},
 		name:      name,
@@ -99,6 +94,14 @@ func (t *TableAlias) Comment() string {
 // Schema implements the Node interface. TableAlias alters the schema of its child element to rename the source of
 // columns to the alias.
 func (t *TableAlias) Schema() sql.Schema {
+	if !t.cachedSch {
+		for i, col := range t.Child.Schema() {
+			newCol := *col
+			newCol.Source = t.name
+			t.sch[i] = &newCol
+		}
+		t.cachedSch = true
+	}
 	return t.sch
 }