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 (
    4651, 5190, 1580, 3324, 3495, 5253, 5387, 
    3321, 3327, 1667, 3932, 2090, 2277, 
    5843, 2278, 5969, 3338, 2400, 5814, 
    5251, 5831, 4239, 5751, 5740, 4191, 
    5839, 2179, 5813, 1629, 5205, 5266, 
    1666, 5024, 5262, 5282, 5565, 3332, 
    5275, 5264, 5825, 5924, 5283, 5838, 
    5268, 5738, 1630, 3494, 5274, 2183, 
    2237, 5273, 5291, 2087, 3491, 3930, 
    4233, 5927, 2239, 8813, 1843, 5182, 
    2092, 5267, 5279, 2176, 2238, 5269, 
    5280, 5017, 1671, 3801, 2091, 1845, 
    2089, 3498, 5286, 2276, 4475, 3506, 
    5211
  ) 
GROUP BY 
  p.product_id

Query time 0.00119

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "2.31"
    },
    "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": 13,
            "rows_produced_per_join": 1,
            "filtered": "10.00",
            "cost_info": {
              "read_cost": "1.70",
              "eval_cost": "0.13",
              "prefix_cost": "1.83",
              "data_read_per_join": "582"
            },
            "used_columns": [
              "post_id",
              "thread_id",
              "status"
            ],
            "attached_condition": "(`softwarepirmam_hewadelivard_cscart_4`.`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": [
              "softwarepirmam_hewadelivard_cscart_4.dp.thread_id"
            ],
            "rows_examined_per_scan": 1,
            "rows_produced_per_join": 0,
            "filtered": "5.00",
            "cost_info": {
              "read_cost": "0.33",
              "eval_cost": "0.01",
              "prefix_cost": "2.28",
              "data_read_per_join": "1"
            },
            "used_columns": [
              "thread_id",
              "object_id",
              "object_type"
            ],
            "attached_condition": "((`softwarepirmam_hewadelivard_cscart_4`.`cscart_discussion`.`object_id` in (4651,5190,1580,3324,3495,5253,5387,3321,3327,1667,3932,2090,2277,5843,2278,5969,3338,2400,5814,5251,5831,4239,5751,5740,4191,5839,2179,5813,1629,5205,5266,1666,5024,5262,5282,5565,3332,5275,5264,5825,5924,5283,5838,5268,5738,1630,3494,5274,2183,2237,5273,5291,2087,3491,3930,4233,5927,2239,8813,1843,5182,2092,5267,5279,2176,2238,5269,5280,5017,1671,3801,2091,1845,2089,3498,5286,2276,4475,3506,5211)) and (`softwarepirmam_hewadelivard_cscart_4`.`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": [
              "softwarepirmam_hewadelivard_cscart_4.cscart_discussion.object_id"
            ],
            "rows_examined_per_scan": 1,
            "rows_produced_per_join": 0,
            "filtered": "100.00",
            "using_index": true,
            "cost_info": {
              "read_cost": "0.02",
              "eval_cost": "0.01",
              "prefix_cost": "2.31",
              "data_read_per_join": "294"
            },
            "used_columns": [
              "product_id"
            ]
          }
        }
      ]
    }
  }
}