have explain format=tree,debug,estimates print an explain tree (#2856)

Author: jycor

enginetest/queries/queries.go

@@ -10160,6 +10160,34 @@ from typestable`,
 			{false},
 		},
 	},
+
+	{
+		Query:            "explain select 1",
+		SkipServerEngine: true,
+		Expected: []sql.Row{
+			{1, "SELECT", "NULL", "NULL", "NULL", "NULL", "NULL", "NULL", "NULL", "NULL", "NULL", ""},
+		},
+	},
+	{
+		Query:            "explain plan select 1",
+		SkipServerEngine: true,
+		Expected: []sql.Row{
+			{"Project"},
+			{" ├─ columns: [1]"},
+			{" └─ Table"},
+			{"     └─ name: "},
+		},
+	},
+	{
+		Query:            "explain format=tree select 1",
+		SkipServerEngine: true,
+		Expected: []sql.Row{
+			{"Project"},
+			{" ├─ columns: [1]"},
+			{" └─ Table"},
+			{"     └─ name: "},
+		},
+	},
 }
 
 var KeylessQueries = []QueryTest{

sql/planbuilder/explain.go

@@ -35,11 +35,15 @@ func (b *Builder) buildExplain(inScope *scope, n *sqlparser.Explain) (outScope *
 	formatFlags := strings.Split(n.ExplainFormat, "_")
 	for _, flag := range formatFlags {
 		switch strings.ToLower(flag) {
-		case "", sqlparser.TreeStr:
-		// tree format, do nothing
+		case "":
+			// no-op
+		case sqlparser.TreeStr:
+			describeOptions.Plan = true
 		case "debug":
+			describeOptions.Plan = true
 			describeOptions.Debug = true
 		case "estimates":
+			describeOptions.Plan = true
 			describeOptions.Estimates = true
 		default:
 			err := errInvalidDescribeFormat.New(n.ExplainFormat, "tree")