SELECT 
  profile_fields_data.field_id, 
  profile_fields_data.value, 
  profile_fields.field_type 
FROM 
  cscart_profile_fields_data AS profile_fields_data 
  LEFT JOIN cscart_profile_fields AS profile_fields ON profile_fields_data.field_id = profile_fields.field_id 
WHERE 
  object_id = 348710 
  AND object_type = 'U'

Query time 0.00028

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "16.03"
    },
    "nested_loop": [
      {
        "table": {
          "table_name": "profile_fields_data",
          "access_type": "ref",
          "possible_keys": [
            "idx_entire_row",
            "idx_obj_objtyp"
          ],
          "key": "idx_obj_objtyp",
          "used_key_parts": [
            "object_id",
            "object_type"
          ],
          "key_length": "6",
          "ref": [
            "const",
            "const"
          ],
          "rows_examined_per_scan": 28,
          "rows_produced_per_join": 28,
          "filtered": "100.00",
          "using_index": true,
          "cost_info": {
            "read_cost": "3.43",
            "eval_cost": "2.80",
            "prefix_cost": "6.23",
            "data_read_per_join": "21K"
          },
          "used_columns": [
            "object_id",
            "object_type",
            "field_id",
            "value"
          ]
        }
      },
      {
        "table": {
          "table_name": "profile_fields",
          "access_type": "eq_ref",
          "possible_keys": [
            "PRIMARY"
          ],
          "key": "PRIMARY",
          "used_key_parts": [
            "field_id"
          ],
          "key_length": "3",
          "ref": [
            "cscart_db.profile_fields_data.field_id"
          ],
          "rows_examined_per_scan": 1,
          "rows_produced_per_join": 28,
          "filtered": "100.00",
          "cost_info": {
            "read_cost": "7.00",
            "eval_cost": "2.80",
            "prefix_cost": "16.03",
            "data_read_per_join": "28K"
          },
          "used_columns": [
            "field_id",
            "field_type"
          ]
        }
      }
    ]
  }
}