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, 1146, 1145, 1144, 1143, 856, 
    1542, 1543, 2258, 2257, 2256, 2255, 
    2265, 2264, 2263, 2253, 2363, 2364, 
    2365, 2559, 2560, 3663, 2587, 2588, 
    2589, 2590, 2603, 2609, 2610, 2611, 
    2729, 2730, 2731, 2732, 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, 3090, 3091, 3092, 
    3093, 3094, 3136, 3137, 3138, 3140, 
    3141
  ) 
GROUP BY 
  p.product_id

Query time 0.00085

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 (908,134,1146,1145,1144,1143,856,1542,1543,2258,2257,2256,2255,2265,2264,2263,2253,2363,2364,2365,2559,2560,3663,2587,2588,2589,2590,2603,2609,2610,2611,2729,2730,2731,2732,2746,2747,2749,2750,2751,2752,2753,2754,2755,2756,2757,2761,2762,2763,2764,2822,2823,2824,2825,2826,2842,2843,2844,2845,2846,2847,2854,2855,2856,2857,2858,2859,2927,2928,2929,3090,3091,3092,3093,3094,3136,3137,3138,3140,3141)) 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"
            ]
          }
        }
      ]
    }
  }
}