don't erase dual table projection (#2889)

Author: jycor

enginetest/queries/queries.go

@@ -10171,6 +10171,18 @@ from typestable`,
 			{2},
 		},
 	},
+	{
+		Query: "select ''",
+		Expected: []sql.Row{
+			{""},
+		},
+	},
+	{
+		Query: "select '' from dual",
+		Expected: []sql.Row{
+			{""},
+		},
+	},
 
 	{
 		Query: "select @@sql_mode = 1",

sql/analyzer/optimization_rules.go

@@ -17,6 +17,8 @@ package analyzer
 import (
 	"strings"
 
+	"github.com/dolthub/go-mysql-server/memory"
+
 	"github.com/dolthub/go-mysql-server/sql"
 	"github.com/dolthub/go-mysql-server/sql/expression"
 	"github.com/dolthub/go-mysql-server/sql/plan"
@@ -43,7 +45,9 @@ func eraseProjection(ctx *sql.Context, a *Analyzer, node sql.Node, scope *plan.S
 	return transform.Node(node, func(node sql.Node) (sql.Node, transform.TreeIdentity, error) {
 		project, ok := node.(*plan.Project)
 		if ok {
-			if project.Schema().CaseSensitiveEquals(project.Child.Schema()) {
+			projSch := project.Schema()
+			childSch := project.Child.Schema()
+			if projSch.CaseSensitiveEquals(childSch) && !childSch.Equals(memory.DualTableSchema.Schema) {
 				a.Log("project erased")
 				return project.Child, transform.NewTree, nil
 			}