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 (
    391789, 265231, 81619, 285940, 81632, 
    271489, 81662, 339530, 265039, 265034, 
    285937, 271497, 387095, 81615, 392212, 
    285942, 339446, 271493, 81639, 271504, 
    265258, 81706, 265031, 339514, 264987, 
    265041, 265261, 265263, 339551, 271500, 
    339521, 286476, 81649, 264988, 285945, 
    271506, 265244, 265256, 271496, 271492, 
    339501, 151897, 339499, 285939, 339495, 
    339721, 366887, 392150, 81622, 392214, 
    392215, 265038, 81652, 271498, 81697, 
    339494, 265242, 339503, 339531, 224456, 
    81693, 339516, 285938, 81660, 271510, 
    359818, 285943, 271503, 339539, 285941, 
    339447, 339509, 81669, 271494, 264989, 
    285946, 264990, 271502, 265232, 265254, 
    339636, 285966, 285970, 114755, 339637, 
    339492, 285968, 339538, 369244, 271490, 
    339506, 265250, 265001, 265237, 271505, 
    339724, 392179, 339635, 81628, 339502
  ) 
GROUP BY 
  p.product_id

Query time 0.00134

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 (391789,265231,81619,285940,81632,271489,81662,339530,265039,265034,285937,271497,387095,81615,392212,285942,339446,271493,81639,271504,265258,81706,265031,339514,264987,265041,265261,265263,339551,271500,339521,286476,81649,264988,285945,271506,265244,265256,271496,271492,339501,151897,339499,285939,339495,339721,366887,392150,81622,392214,392215,265038,81652,271498,81697,339494,265242,339503,339531,224456,81693,339516,285938,81660,271510,359818,285943,271503,339539,285941,339447,339509,81669,271494,264989,285946,264990,271502,265232,265254,339636,285966,285970,114755,339637,339492,285968,339538,369244,271490,339506,265250,265001,265237,271505,339724,392179,339635,81628,339502)) 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"
            ]
          }
        }
      ]
    }
  }
}