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 (
    2467, 2468, 2469, 2470, 2471, 2472, 2473, 
    2475, 2476, 2477, 2478, 2483, 2482, 
    2484, 2485, 2486, 2487, 2488, 2489, 
    2490, 2491, 2492, 2493, 2494, 2495, 
    2496, 2497, 2499, 2500, 2503, 2504, 
    2505, 2517, 2518, 2519, 2524, 2525, 
    2526, 2527, 2528, 2529, 2530, 2531, 
    2538, 2540, 2541, 2550, 2549, 2548, 
    2544, 2551, 2552, 2534, 2564, 2565, 
    2576, 2577, 2578, 2579, 2580, 2581, 
    2585, 2586, 2596, 2597, 2598, 2599, 
    2600, 2601, 2602, 2604, 2605, 2606, 
    2607, 2608, 2615, 2620, 2621, 2623, 
    2624, 2625, 2626, 2631, 2632, 2633, 
    2685, 2686, 2687, 2502, 3639, 8417, 
    8418, 8419, 8420, 8398, 8415, 8416, 
    8421, 8399, 8476, 8477, 10140, 10141, 
    10142, 10143, 10144, 10145, 10224, 
    10223, 10222, 10228, 10229, 10230, 
    10233, 10234
  ) 
GROUP BY 
  p.product_id

Query time 0.00114

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 (2467,2468,2469,2470,2471,2472,2473,2475,2476,2477,2478,2483,2482,2484,2485,2486,2487,2488,2489,2490,2491,2492,2493,2494,2495,2496,2497,2499,2500,2503,2504,2505,2517,2518,2519,2524,2525,2526,2527,2528,2529,2530,2531,2538,2540,2541,2550,2549,2548,2544,2551,2552,2534,2564,2565,2576,2577,2578,2579,2580,2581,2585,2586,2596,2597,2598,2599,2600,2601,2602,2604,2605,2606,2607,2608,2615,2620,2621,2623,2624,2625,2626,2631,2632,2633,2685,2686,2687,2502,3639,8417,8418,8419,8420,8398,8415,8416,8421,8399,8476,8477,10140,10141,10142,10143,10144,10145,10224,10223,10222,10228,10229,10230,10233,10234)) 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"
            ]
          }
        }
      ]
    }
  }
}