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 (
    431129, 435509, 430656, 430863, 431338, 
    431423, 430856, 430993, 431051, 431074, 
    431165, 339782, 430688, 430702, 430722, 
    430723, 430778, 430816, 430984, 431247, 
    339718, 339675, 431005, 431027, 430627, 
    430742, 430805, 431007, 431022, 431184, 
    339604, 430720, 430839, 430878, 430933, 
    430994, 431177, 430826, 430991, 431019, 
    431357, 430917, 430990, 431108, 431264, 
    431275, 431426, 430872, 430889, 430897, 
    430918, 430940, 431297, 431345, 430690, 
    430791, 430840, 431220, 431501, 430652, 
    430790, 430835, 430958, 430968, 430977, 
    430979, 431342, 430674, 430804, 430828, 
    430886, 430960, 431138, 431358, 430786, 
    430871, 430969, 430931, 430952, 430972, 
    431006, 431428, 431451, 431467, 339597, 
    430780, 431288, 431336, 431395, 430692, 
    430715, 430770, 430894, 430953, 431503, 
    339846, 387050, 430666, 430726, 431037
  ) 
GROUP BY 
  p.product_id

Query time 0.00099

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 (431129,435509,430656,430863,431338,431423,430856,430993,431051,431074,431165,339782,430688,430702,430722,430723,430778,430816,430984,431247,339718,339675,431005,431027,430627,430742,430805,431007,431022,431184,339604,430720,430839,430878,430933,430994,431177,430826,430991,431019,431357,430917,430990,431108,431264,431275,431426,430872,430889,430897,430918,430940,431297,431345,430690,430791,430840,431220,431501,430652,430790,430835,430958,430968,430977,430979,431342,430674,430804,430828,430886,430960,431138,431358,430786,430871,430969,430931,430952,430972,431006,431428,431451,431467,339597,430780,431288,431336,431395,430692,430715,430770,430894,430953,431503,339846,387050,430666,430726,431037)) 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"
            ]
          }
        }
      ]
    }
  }
}