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 (
    908, 134, 1142, 853, 1146, 1145, 1144, 
    1143, 856, 1540, 1541, 1542, 1543, 1544, 
    1545, 2258, 2257, 2256, 2255, 2260, 
    2259, 2251, 2265, 2264, 2263, 2253, 
    2363, 2364, 2365, 2373, 2374, 2559, 
    2560, 3663, 2587, 2588, 2589, 2590, 
    2603, 2609, 2610, 2611, 2696, 2697, 
    2695, 2694, 2723, 2724, 2725, 2726, 
    2729, 2730, 2731, 2732, 2742, 2743, 
    2744, 2745, 2746, 2747, 2749, 2750, 
    2751, 2752, 2753, 2754, 2755, 2756, 
    2757, 2761, 2762, 2763, 2764, 2822, 
    2823, 2824, 2825, 2826, 2842, 2843, 
    2844, 2845, 2846, 2847, 2854, 2855, 
    2856, 2857, 2858, 2859, 2927, 2928, 
    2929, 3026, 3030, 3029, 3090, 3091, 
    3092, 3093, 3094, 3096, 3097, 3098, 
    3136, 3137, 3138, 3140, 3141, 3142, 
    3610, 3556, 3557
  ) 
GROUP BY 
  p.product_id

Query time 0.00102

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 (908,134,1142,853,1146,1145,1144,1143,856,1540,1541,1542,1543,1544,1545,2258,2257,2256,2255,2260,2259,2251,2265,2264,2263,2253,2363,2364,2365,2373,2374,2559,2560,3663,2587,2588,2589,2590,2603,2609,2610,2611,2696,2697,2695,2694,2723,2724,2725,2726,2729,2730,2731,2732,2742,2743,2744,2745,2746,2747,2749,2750,2751,2752,2753,2754,2755,2756,2757,2761,2762,2763,2764,2822,2823,2824,2825,2826,2842,2843,2844,2845,2846,2847,2854,2855,2856,2857,2858,2859,2927,2928,2929,3026,3030,3029,3090,3091,3092,3093,3094,3096,3097,3098,3136,3137,3138,3140,3141,3142,3610,3556,3557)) 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"
            ]
          }
        }
      ]
    }
  }
}