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 (
    339776, 339474, 339552, 339649, 339743, 
    339756, 339647, 339665, 339470, 339773, 
    339577, 339848, 339703, 339749, 339669, 
    339455, 339666, 339789, 339651, 339565, 
    339544, 339626, 339670, 339543, 339646, 
    339824, 339833, 339834, 339575, 339784, 
    339698, 339830, 339631, 339628, 339662, 
    339767, 339557, 339567, 339549, 339595, 
    339823, 339673, 339853, 339658, 339717, 
    339763, 339852, 339542, 339818, 339653, 
    339487, 339822, 339472, 339613, 339682, 
    339480, 339555, 339639, 339560, 339579, 
    339680, 339559, 339788, 339850, 339854, 
    339815, 339772, 339561, 339785, 339766, 
    339475, 339688, 339566, 339661, 339558, 
    339819, 339585, 339837, 339563, 339640, 
    339568, 339790, 339578, 339634, 339598, 
    339629, 339672, 339546, 339844, 339641, 
    339832, 339840, 339624, 339627, 339687, 
    339842, 339612, 339827, 339642, 339800
  ) 
GROUP BY 
  p.product_id

Query time 0.00192

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "7.29"
    },
    "grouping_operation": {
      "using_temporary_table": true,
      "using_filesort": true,
      "cost_info": {
        "sort_cost": "0.10"
      },
      "nested_loop": [
        {
          "table": {
            "table_name": "dp",
            "access_type": "index",
            "possible_keys": [
              "thread_id",
              "thread_id_2"
            ],
            "key": "thread_id_2",
            "used_key_parts": [
              "thread_id",
              "status"
            ],
            "key_length": "6",
            "rows_examined_per_scan": 19,
            "rows_produced_per_join": 1,
            "filtered": "10.00",
            "using_index": true,
            "cost_info": {
              "read_cost": "4.42",
              "eval_cost": "0.38",
              "prefix_cost": "4.80",
              "data_read_per_join": "851"
            },
            "used_columns": [
              "post_id",
              "thread_id",
              "status"
            ],
            "attached_condition": "(`webmarco`.`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": [
              "webmarco.dp.thread_id"
            ],
            "rows_examined_per_scan": 1,
            "rows_produced_per_join": 0,
            "filtered": "5.00",
            "cost_info": {
              "read_cost": "1.90",
              "eval_cost": "0.02",
              "prefix_cost": "7.08",
              "data_read_per_join": "2"
            },
            "used_columns": [
              "thread_id",
              "object_id",
              "object_type"
            ],
            "attached_condition": "((`webmarco`.`cscart_discussion`.`object_id` in (339776,339474,339552,339649,339743,339756,339647,339665,339470,339773,339577,339848,339703,339749,339669,339455,339666,339789,339651,339565,339544,339626,339670,339543,339646,339824,339833,339834,339575,339784,339698,339830,339631,339628,339662,339767,339557,339567,339549,339595,339823,339673,339853,339658,339717,339763,339852,339542,339818,339653,339487,339822,339472,339613,339682,339480,339555,339639,339560,339579,339680,339559,339788,339850,339854,339815,339772,339561,339785,339766,339475,339688,339566,339661,339558,339819,339585,339837,339563,339640,339568,339790,339578,339634,339598,339629,339672,339546,339844,339641,339832,339840,339624,339627,339687,339842,339612,339827,339642,339800)) and (`webmarco`.`cscart_discussion`.`object_type` = 'P'))"
          }
        },
        {
          "table": {
            "table_name": "p",
            "access_type": "eq_ref",
            "possible_keys": [
              "PRIMARY",
              "age_verification",
              "status",
              "idx_parent_product_id",
              "idx_master_product_id"
            ],
            "key": "PRIMARY",
            "used_key_parts": [
              "product_id"
            ],
            "key_length": "3",
            "ref": [
              "webmarco.cscart_discussion.object_id"
            ],
            "rows_examined_per_scan": 1,
            "rows_produced_per_join": 0,
            "filtered": "100.00",
            "using_index": true,
            "cost_info": {
              "read_cost": "0.10",
              "eval_cost": "0.02",
              "prefix_cost": "7.19",
              "data_read_per_join": "760"
            },
            "used_columns": [
              "product_id"
            ]
          }
        }
      ]
    }
  }
}