SELECT 
  cscart_discussion_rating.* 
FROM 
  cscart_discussion_rating 
  LEFT JOIN cscart_discussion_posts ON cscart_discussion_posts.post_id = cscart_discussion_rating.post_id 
WHERE 
  1 
  AND cscart_discussion_rating.thread_id = 247917 
  AND cscart_discussion_posts.status = 'A' 
ORDER BY 
  cscart_discussion_rating.rating_value DESC

Query time 0.00047

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "2.40"
    },
    "ordering_operation": {
      "using_filesort": true,
      "nested_loop": [
        {
          "table": {
            "table_name": "cscart_discussion_rating",
            "access_type": "ref",
            "possible_keys": [
              "PRIMARY",
              "thread_id"
            ],
            "key": "thread_id",
            "used_key_parts": [
              "thread_id"
            ],
            "key_length": "3",
            "ref": [
              "const"
            ],
            "rows_examined_per_scan": 1,
            "rows_produced_per_join": 1,
            "filtered": "100.00",
            "index_condition": "(`webmarco`.`cscart_discussion_rating`.`thread_id` <=> 247917)",
            "cost_info": {
              "read_cost": "1.00",
              "eval_cost": "0.20",
              "prefix_cost": "1.20",
              "data_read_per_join": "16"
            },
            "used_columns": [
              "rating_value",
              "post_id",
              "thread_id"
            ]
          }
        },
        {
          "table": {
            "table_name": "cscart_discussion_posts",
            "access_type": "eq_ref",
            "possible_keys": [
              "PRIMARY"
            ],
            "key": "PRIMARY",
            "used_key_parts": [
              "post_id"
            ],
            "key_length": "3",
            "ref": [
              "webmarco.cscart_discussion_rating.post_id"
            ],
            "rows_examined_per_scan": 1,
            "rows_produced_per_join": 0,
            "filtered": "10.00",
            "cost_info": {
              "read_cost": "1.00",
              "eval_cost": "0.02",
              "prefix_cost": "2.40",
              "data_read_per_join": "44"
            },
            "used_columns": [
              "post_id",
              "status"
            ],
            "attached_condition": "(`webmarco`.`cscart_discussion_posts`.`status` = 'A')"
          }
        }
      ]
    }
  }
}