    "access_type": "ref",
      "possible_keys": ["i_o_custkey"],
      "key": "i_o_custkey",
      "key_length": "5",
      "used_key_parts": ["o_custkey"],
      "ref": ["dbt3sf1.customer.c_custkey"],
      "r_loops": 150000,
      "rows": 7,
      "r_rows": 10,
      "r_total_time_ms": 39208,
      "filtered": 100,
      "r_filtered": 100
     }
    }
   }
  ]
 }
}
ANALYZE shows that 39.2 seconds were spent in the subquery, which was executed
150K times (for every row of outer table).

URL: https://mariadb.com/kb/en/analyze-formatjson-examples/https://mariadb.com/kb/en/analyze-formatjson-examples/