EXPLAIN FORMAT=JSON: Support range+MRR plans (when MRR is used but BKA is not)

mysql-test/r/explain_json.result

@@ -665,4 +665,31 @@ EXPLAIN
 }
 set optimizer_switch=@tmp;
 drop table t1,t2;
+#
+# MRR for range access (no BKA, just MRR)
+#
+create table t1 (a int, b int, key(a));
+insert into t1 select tbl1.a+10*tbl2.a, 12345 from t0 tbl1, t0 tbl2;
+set @tmp= @@optimizer_switch;
+set optimizer_switch='mrr=on,mrr_sort_keys=on';
+explain format=json select * from t1 where a < 3;
+EXPLAIN
+{
+  "query_block": {
+    "select_id": 1,
+    "table": {
+      "table_name": "t1",
+      "access_type": "range",
+      "possible_keys": ["a"],
+      "key": "a",
+      "key_length": "5",
+      "used_key_parts": ["a"],
+      "rows": 1,
+      "filtered": 100,
+      "index_condition": "(t1.a < 3)",
+      "mrr_type": "Rowid-ordered scan"
+    }
+  }
+}
+drop table t1;
 drop table t0;

mysql-test/t/explain_json.test

@@ -134,5 +134,17 @@ select * from t2 where t2.a in ( select a from t1 where t1.b=t2.b);
 set optimizer_switch=@tmp;
 
 drop table t1,t2;
+
+--echo #
+--echo # MRR for range access (no BKA, just MRR)
+--echo #
+create table t1 (a int, b int, key(a));
+insert into t1 select tbl1.a+10*tbl2.a, 12345 from t0 tbl1, t0 tbl2;
+set @tmp= @@optimizer_switch;
+set optimizer_switch='mrr=on,mrr_sort_keys=on';
+
+explain format=json select * from t1 where a < 3;
+
+drop table t1;
 drop table t0;
 

sql/sql_explain.cc

@@ -1144,6 +1144,9 @@ void Explain_table_access::tag_to_json(Json_writer *writer, enum explain_extra_t
     case ET_LOOSESCAN:
       writer->add_member("loose_scan").add_bool(true);
       break;
+    case ET_USING_MRR:
+      writer->add_member("mrr_type").add_str(mrr_type.c_ptr());
+      break;
     default:
       DBUG_ASSERT(0);
   }

sql/sql_explain.h

@@ -450,13 +450,24 @@ enum explain_extra_tag
 };
 
 
+/*
+  Explain data structure describing join buffering use.
+*/
+
 class EXPLAIN_BKA_TYPE
 {
 public:
   EXPLAIN_BKA_TYPE() : join_alg(NULL) {}
 
   bool incremental;
+
+  /* 
+    NULL if no join buferring used.
+    Other values: BNL, BNLH, BKA, BKAH.
+  */
   const char *join_alg;
+
+  /* Information about MRR usage.  */
   StringBuffer<64> mrr_type;
   
   bool is_using_jbuf() { return (join_alg != NULL); }