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 (
    263925, 264760, 264761, 264762, 264763, 
    263981, 264764, 264765, 264766, 264767, 
    264208, 264847, 264848, 264849, 264850, 
    264851, 264852, 264853, 264854, 264855, 
    264219, 264909, 264910, 264911, 264912, 
    264913, 264914, 264915, 264916, 264917, 
    264193, 264918, 264919, 264920, 264921, 
    264922, 264923, 264924, 264925, 264926, 
    264156, 264927, 264928, 264929, 264930, 
    264931, 264932, 264933, 264934, 264935, 
    264950, 264951, 264952, 264953, 264954, 
    263720, 264936, 264937, 264938, 264939, 
    264940, 264941, 264942, 264943, 264944, 
    264945, 264946, 264947, 264948, 264949, 
    264955, 264956, 264957, 264958, 264959, 
    264102, 264969, 264970, 264971, 264972, 
    264973, 264974, 264975, 264976, 264977
  ) 
GROUP BY 
  p.product_id

Query time 0.00125

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 (263925,264760,264761,264762,264763,263981,264764,264765,264766,264767,264208,264847,264848,264849,264850,264851,264852,264853,264854,264855,264219,264909,264910,264911,264912,264913,264914,264915,264916,264917,264193,264918,264919,264920,264921,264922,264923,264924,264925,264926,264156,264927,264928,264929,264930,264931,264932,264933,264934,264935,264950,264951,264952,264953,264954,263720,264936,264937,264938,264939,264940,264941,264942,264943,264944,264945,264946,264947,264948,264949,264955,264956,264957,264958,264959,264102,264969,264970,264971,264972,264973,264974,264975,264976,264977)) 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"
            ]
          }
        }
      ]
    }
  }
}