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 (
    430938, 431208, 431241, 431335, 431447, 
    430727, 430973, 430985, 431292, 430752, 
    430757, 430942, 431383, 431488, 431502, 
    430978, 431017, 431210, 431236, 431330, 
    430719, 430929, 431227, 431235, 431427, 
    431466, 430630, 430632, 431263, 431482, 
    430943, 431040, 430955, 431415, 431018, 
    431069, 431364, 430693, 430971, 431361, 
    431378, 381708, 431281, 431421, 431302, 
    431385, 431285, 431369, 430717, 430785, 
    431212, 431402, 430725, 430833, 430941, 
    431086, 430736, 430987, 431384, 430724, 
    430733, 431229, 431300, 431377, 431472, 
    430721, 431223, 431253, 431181, 431454, 
    430682, 430951, 430956, 431081, 431431, 
    431225, 431468, 381715, 431387, 431446, 
    430716, 431401, 431438, 431143, 431382, 
    431412, 431430, 431058, 431170, 430945, 
    431215, 431460, 431224, 431294, 431310, 
    431312, 431367, 431450, 339768, 430731
  ) 
GROUP BY 
  p.product_id

Query time 0.00158

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 (430938,431208,431241,431335,431447,430727,430973,430985,431292,430752,430757,430942,431383,431488,431502,430978,431017,431210,431236,431330,430719,430929,431227,431235,431427,431466,430630,430632,431263,431482,430943,431040,430955,431415,431018,431069,431364,430693,430971,431361,431378,381708,431281,431421,431302,431385,431285,431369,430717,430785,431212,431402,430725,430833,430941,431086,430736,430987,431384,430724,430733,431229,431300,431377,431472,430721,431223,431253,431181,431454,430682,430951,430956,431081,431431,431225,431468,381715,431387,431446,430716,431401,431438,431143,431382,431412,431430,431058,431170,430945,431215,431460,431224,431294,431310,431312,431367,431450,339768,430731)) 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"
            ]
          }
        }
      ]
    }
  }
}