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 (
    12498, 15432, 3317, 3314, 3319, 3316, 
    3320, 3318, 3315, 3321, 3322, 3298, 
    3299, 3329, 3330, 3306, 3305, 3304, 
    3325, 3289, 3287, 3283, 3284, 3285, 
    3286, 3292, 3291, 3300, 3301, 3302, 
    3326, 3295, 3294, 3327, 3297, 3307, 
    3309, 3308, 3293, 3288, 3290, 3303, 
    3310, 3324, 3323, 13136, 3922, 15904, 
    3442, 3441, 15431, 4884, 13070, 11475, 
    877, 3500, 3501, 3502, 3504, 3505, 883, 
    884, 3503, 13048, 13038, 3928, 16545, 
    16551, 16528, 15765, 16526, 16547, 
    15767, 16529, 16523, 15764, 16544, 
    16539, 16548, 16527, 16530, 16533, 
    16550, 16538, 16531, 16532, 16546, 
    16520, 16189, 16537, 15876, 16543, 
    15759, 15760, 16534, 15763
  ) 
GROUP BY 
  p.product_id

Query time 0.00098

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 (12498,15432,3317,3314,3319,3316,3320,3318,3315,3321,3322,3298,3299,3329,3330,3306,3305,3304,3325,3289,3287,3283,3284,3285,3286,3292,3291,3300,3301,3302,3326,3295,3294,3327,3297,3307,3309,3308,3293,3288,3290,3303,3310,3324,3323,13136,3922,15904,3442,3441,15431,4884,13070,11475,877,3500,3501,3502,3504,3505,883,884,3503,13048,13038,3928,16545,16551,16528,15765,16526,16547,15767,16529,16523,15764,16544,16539,16548,16527,16530,16533,16550,16538,16531,16532,16546,16520,16189,16537,15876,16543,15759,15760,16534,15763)) 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"
            ]
          }
        }
      ]
    }
  }
}