Optimizer trace: Make ref_optimizer_key_uses[*] show the index name

mysql-test/main/opt_trace.result

@@ -963,12 +963,14 @@ explain select * from t1,t2 where t1.a=t2.b+2 and t2.a= t1.b	{
             "ref_optimizer_key_uses": [
               {
                 "table": "t1",
+                "index": "a",
                 "field": "a",
                 "equals": "t2.b + 2",
                 "null_rejecting": true
               },
               {
                 "table": "t2",
+                "index": "a",
                 "field": "a",
                 "equals": "t1.b",
                 "null_rejecting": true
@@ -1975,18 +1977,21 @@ explain  select * from t1 where a=1 and b=2 order by c limit 1	{
             "ref_optimizer_key_uses": [
               {
                 "table": "t1",
+                "index": "a_c",
                 "field": "a",
                 "equals": "1",
                 "null_rejecting": true
               },
               {
                 "table": "t1",
+                "index": "a_b",
                 "field": "a",
                 "equals": "1",
                 "null_rejecting": true
               },
               {
                 "table": "t1",
+                "index": "a_b",
                 "field": "b",
                 "equals": "2",
                 "null_rejecting": true
@@ -2346,6 +2351,7 @@ select t1.a from t1 left join t2 on t1.a=t2.a	{
             "ref_optimizer_key_uses": [
               {
                 "table": "t2",
+                "index": "PRIMARY",
                 "field": "a",
                 "equals": "t1.a",
                 "null_rejecting": true
@@ -2484,6 +2490,7 @@ explain select * from t1 left join t2 on t2.a=t1.a	{
             "ref_optimizer_key_uses": [
               {
                 "table": "t2",
+                "index": "PRIMARY",
                 "field": "a",
                 "equals": "t1.a",
                 "null_rejecting": true
@@ -2670,24 +2677,28 @@ explain select t1.a from t1 left join (t2 join t3 on t2.b=t3.b) on t2.a=t1.a and
             "ref_optimizer_key_uses": [
               {
                 "table": "t2",
+                "index": "PRIMARY",
                 "field": "a",
                 "equals": "t1.a",
                 "null_rejecting": true
               },
               {
                 "table": "t2",
+                "index": "PRIMARY",
                 "field": "a",
                 "equals": "t3.a",
                 "null_rejecting": true
               },
               {
                 "table": "t3",
+                "index": "PRIMARY",
                 "field": "a",
                 "equals": "t2.a",
                 "null_rejecting": true
               },
               {
                 "table": "t3",
+                "index": "PRIMARY",
                 "field": "a",
                 "equals": "t1.a",
                 "null_rejecting": true
@@ -3187,36 +3198,42 @@ explain select * from t1 where pk = 2 and a=5 and b=1	{
             "ref_optimizer_key_uses": [
               {
                 "table": "t1",
+                "index": "pk",
                 "field": "pk",
                 "equals": "2",
                 "null_rejecting": true
               },
               {
                 "table": "t1",
+                "index": "pk_a",
                 "field": "pk",
                 "equals": "2",
                 "null_rejecting": true
               },
               {
                 "table": "t1",
+                "index": "pk_a",
                 "field": "a",
                 "equals": "5",
                 "null_rejecting": true
               },
               {
                 "table": "t1",
+                "index": "pk_a_b",
                 "field": "pk",
                 "equals": "2",
                 "null_rejecting": true
               },
               {
                 "table": "t1",
+                "index": "pk_a_b",
                 "field": "a",
                 "equals": "5",
                 "null_rejecting": true
               },
               {
                 "table": "t1",
+                "index": "pk_a_b",
                 "field": "b",
                 "equals": "1",
                 "null_rejecting": true
@@ -3867,12 +3884,14 @@ explain delete t0,t1 from t0, t1 where t0.a=t1.a and t1.a<3	{
             "ref_optimizer_key_uses": [
               {
                 "table": "t0",
+                "index": "a",
                 "field": "a",
                 "equals": "t1.a",
                 "null_rejecting": true
               },
               {
                 "table": "t1",
+                "index": "a",
                 "field": "a",
                 "equals": "t0.a",
                 "null_rejecting": true

mysql-test/main/opt_trace_index_merge_innodb.result

@@ -75,6 +75,7 @@ explain select * from t1 where pk1 != 0  and key1 = 1	{
             "ref_optimizer_key_uses": [
               {
                 "table": "t1",
+                "index": "key1",
                 "field": "key1",
                 "equals": "1",
                 "null_rejecting": false

sql/sql_test.cc

@@ -694,6 +694,10 @@ void print_keyuse_array_for_trace(THD *thd, DYNAMIC_ARRAY *keyuse_array)
     KEYUSE *keyuse= (KEYUSE*)dynamic_array_ptr(keyuse_array, i);
     Json_writer_object keyuse_elem(thd);
     keyuse_elem.add_table_name(keyuse->table->reginfo.join_tab);
+    if (keyuse->keypart != FT_KEYPART && !keyuse->is_for_hash_join())
+    {
+      keyuse_elem.add("index", keyuse->table->key_info[keyuse->key].name);
+    }
     keyuse_elem.add("field", (keyuse->keypart == FT_KEYPART) ? "<fulltext>":
                                         (keyuse->is_for_hash_join() ?
                                         keyuse->table->field[keyuse->keypart]