Fix typos in JSON MRR output

Author: spetrunia

mysql-test/r/explain_json.result

@@ -1492,7 +1492,7 @@ EXPLAIN
  "buffer_type": "flat",
  "buffer_size": "256Kb",
  "join_type": "BKA",
- "mrr_type": "; Rowid-ordered scan"
+ "mrr_type": "Rowid-ordered scan"
  }
  }
 }
@@ -1535,7 +1535,7 @@ ANALYZE
  "buffer_type": "flat",
  "buffer_size": "256Kb",
  "join_type": "BKA",
- "mrr_type": "; Rowid-ordered scan",
+ "mrr_type": "Rowid-ordered scan",
  "r_filtered": 100
  }
  }

sql/sql_explain.cc

@@ -1809,7 +1809,10 @@ void Explain_table_access::append_tag_name(String *str, enum explain_extra_tag t
  str->append(STRING_WITH_LEN(" join"));
  str->append(STRING_WITH_LEN(")"));
  if (bka_type.mrr_type.length())
+ {
+ str->append(STRING_WITH_LEN("; "));
  str->append(bka_type.mrr_type);
+ }
 
  break;
  }

sql/sql_join_cache.cc

@@ -2620,7 +2620,8 @@ static void add_mrr_explain_info(String *str, uint mrr_mode, handler *file)
  sizeof(mrr_str_buf));
  if (len > 0)
  {
- str->append(STRING_WITH_LEN("; "));
+ if (str->length())
+ str->append(STRING_WITH_LEN("; "));
  str->append(mrr_str_buf, len);
  }
 }