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 (
    220173, 339556, 254330, 339744, 339676, 
    339576, 339461, 391686, 339748, 392163, 
    387088, 339667, 339645, 151062, 339684, 
    151100, 254331, 339583, 387086, 339588, 
    339708, 387055, 339826, 339454, 339547, 
    339553, 339633, 387090, 339580, 339548, 
    339657, 339777, 339632, 339649, 339711, 
    339743, 339719, 339647, 339716, 339773, 
    339776, 387065, 339474, 339756, 339745, 
    339471, 387054, 265037, 339577, 339848, 
    339666, 339552, 339665, 339669, 339767, 
    339670, 387066, 339833, 339698, 339703, 
    220171, 339557, 339544, 339646, 339651, 
    339749, 339824, 339455, 339628, 339591, 
    387052, 339784, 339549, 339565, 339673, 
    339614, 339789, 339575, 339662, 265040, 
    339595, 339834, 339853, 339653, 339658, 
    339818, 339626, 339631, 339717, 339852, 
    339559, 339470, 339545, 339487, 339542, 
    339822, 339579, 339567, 271697, 339850
  ) 
GROUP BY 
  p.product_id

Query time 0.00102

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 (220173,339556,254330,339744,339676,339576,339461,391686,339748,392163,387088,339667,339645,151062,339684,151100,254331,339583,387086,339588,339708,387055,339826,339454,339547,339553,339633,387090,339580,339548,339657,339777,339632,339649,339711,339743,339719,339647,339716,339773,339776,387065,339474,339756,339745,339471,387054,265037,339577,339848,339666,339552,339665,339669,339767,339670,387066,339833,339698,339703,220171,339557,339544,339646,339651,339749,339824,339455,339628,339591,387052,339784,339549,339565,339673,339614,339789,339575,339662,265040,339595,339834,339853,339653,339658,339818,339626,339631,339717,339852,339559,339470,339545,339487,339542,339822,339579,339567,271697,339850)) 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"
            ]
          }
        }
      ]
    }
  }
}