SELECT 
  p.product_id, 
  ifnull(
    count(dp.post_id), 
    0
  ) as discussion_amount_posts 
FROM 
  cscart_discussion 
  INNER JOIN cscart_products as p ON (
    cscart_discussion.object_id = p.product_id
  ) 
  INNER JOIN cscart_discussion_posts as dp ON (
    cscart_discussion.thread_id = dp.thread_id 
    AND cscart_discussion.object_type = 'P'
  ) 
WHERE 
  dp.status = 'A' 
  and p.product_id in (
    4343, 2380, 11140, 2424, 2426, 2428, 
    2429, 2430, 2431, 16509, 2432, 2438, 
    17194, 2439, 2440, 15941, 2441, 13684, 
    17173, 13827, 2443, 12952, 2447, 2434, 
    2446, 2445, 16235, 2444, 13492, 4340, 
    2364, 13765, 13826, 12953, 13822, 4378, 
    16952, 15899, 15900, 15792, 15784, 
    12955, 12954, 2451, 2453, 11894, 11893, 
    2457, 2458, 2460, 17184, 2459, 2461, 
    11138, 2365, 16580, 2360, 16138, 4337, 
    2366, 4342, 16222
  ) 
GROUP BY 
  p.product_id

Query time 0.00090

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "1.69"
    },
    "grouping_operation": {
      "using_temporary_table": true,
      "using_filesort": false,
      "nested_loop": [
        {
          "table": {
            "table_name": "dp",
            "access_type": "ALL",
            "possible_keys": [
              "thread_id",
              "thread_id_2"
            ],
            "rows_examined_per_scan": 8,
            "rows_produced_per_join": 1,
            "filtered": "12.50",
            "cost_info": {
              "read_cost": "1.22",
              "eval_cost": "0.10",
              "prefix_cost": "1.32",
              "data_read_per_join": "448"
            },
            "used_columns": [
              "post_id",
              "thread_id",
              "status"
            ],
            "attached_condition": "(`dev_yourmart_co_bw`.`dp`.`status` = 'A')"
          }
        },
        {
          "table": {
            "table_name": "cscart_discussion",
            "access_type": "eq_ref",
            "possible_keys": [
              "PRIMARY",
              "object_id"
            ],
            "key": "PRIMARY",
            "used_key_parts": [
              "thread_id"
            ],
            "key_length": "3",
            "ref": [
              "dev_yourmart_co_bw.dp.thread_id"
            ],
            "rows_examined_per_scan": 1,
            "rows_produced_per_join": 0,
            "filtered": "5.00",
            "cost_info": {
              "read_cost": "0.25",
              "eval_cost": "0.01",
              "prefix_cost": "1.67",
              "data_read_per_join": "1"
            },
            "used_columns": [
              "thread_id",
              "object_id",
              "object_type"
            ],
            "attached_condition": "((`dev_yourmart_co_bw`.`cscart_discussion`.`object_id` in (4343,2380,11140,2424,2426,2428,2429,2430,2431,16509,2432,2438,17194,2439,2440,15941,2441,13684,17173,13827,2443,12952,2447,2434,2446,2445,16235,2444,13492,4340,2364,13765,13826,12953,13822,4378,16952,15899,15900,15792,15784,12955,12954,2451,2453,11894,11893,2457,2458,2460,17184,2459,2461,11138,2365,16580,2360,16138,4337,2366,4342,16222)) and (`dev_yourmart_co_bw`.`cscart_discussion`.`object_type` = 'P'))"
          }
        },
        {
          "table": {
            "table_name": "p",
            "access_type": "eq_ref",
            "possible_keys": [
              "PRIMARY"
            ],
            "key": "PRIMARY",
            "used_key_parts": [
              "product_id"
            ],
            "key_length": "3",
            "ref": [
              "dev_yourmart_co_bw.cscart_discussion.object_id"
            ],
            "rows_examined_per_scan": 1,
            "rows_produced_per_join": 0,
            "filtered": "100.00",
            "using_index": true,
            "cost_info": {
              "read_cost": "0.01",
              "eval_cost": "0.01",
              "prefix_cost": "1.69",
              "data_read_per_join": "266"
            },
            "used_columns": [
              "product_id"
            ]
          }
        }
      ]
    }
  }
}