fix and some tests

Author: James Cor

enginetest/queries/script_queries.go

@@ -8506,6 +8506,59 @@ where
 			},
 		},
 	},
+	{
+		Name:    "merge join optimization",
+		Dialect: "mysql",
+		SetUpScript: []string{
+			"create table t1 (i int primary key);",
+			"create table t2 (j int primary key);",
+			"insert into t1 values (1), (2), (3);",
+			"insert into t2 values (2), (3), (4);",
+
+			"create table t3 (i int, j int, primary key (i, j));",
+			"create table t4 (x int, y int, primary key (x, y));",
+			"insert into t3 values (1, 1), (2, 2), (3, 3);",
+			"insert into t4 values (2, 2), (3, 3), (4, 4);",
+		},
+		Assertions: []ScriptTestAssertion{
+			{
+				Query: "select /*+ MERGE_JOIN(t1, t2) */ * from t1 join t2 on t1.i = t2.j order by t1.i;",
+				Expected: []sql.Row{
+					{2, 2},
+					{3, 3},
+				},
+			},
+			{
+				Query: "select /*+ MERGE_JOIN(t1, t2) */ * from t1 join t2 on t1.i = t2.j order by t2.j;",
+				Expected: []sql.Row{
+					{2, 2},
+					{3, 3},
+				},
+			},
+			{
+				Query: "select /*+ MERGE_JOIN(t1, t2) */ * from t1 join t2 on t1.i = t2.j order by t1.i desc;",
+				Expected: []sql.Row{
+					{3, 3},
+					{2, 2},
+				},
+			},
+			{
+				Query: "select /*+ MERGE_JOIN(t1, t2) */ * from t1 join t2 on t1.i = t2.j order by t2.j desc;",
+				Expected: []sql.Row{
+					{3, 3},
+					{2, 2},
+				},
+			},
+
+			{
+				Query: "explain plan select /*+ MERGE_JOIN(t3, t4) */ * from t3 join t3 on t3.i = t4.j order by t3.i;",
+				Expected: []sql.Row{
+					{2, 2},
+					{3, 3},
+				},
+			},
+		},
+	},
 
 	// Char tests
 	{

sql/analyzer/replace_sort.go

@@ -189,7 +189,7 @@ func replaceIdxSortHelper(ctx *sql.Context, scope *plan.Scope, node sql.Node, so
 				if sameLeft {
 					newLeft, reversible, err = buildReverseIndexedTable(ctx, newLeft)
 				} else {
-					newRight, reversible, err = buildReverseIndexedTable(ctx, newLeft)
+					newRight, reversible, err = buildReverseIndexedTable(ctx, newRight)
 				}
 				if err != nil {
 					return nil, transform.SameTree, err