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 (
    5696, 5621, 5892, 5719, 5706, 5720, 5703, 
    4894, 4962, 5691, 5687, 3733, 4915, 
    3589, 4976, 3811, 6093, 6194, 4935, 
    3734, 3641, 5700, 6064, 3798, 4931, 
    3730, 3638, 3729, 3851, 6204, 3942, 
    3732, 3448, 3728, 3852, 3850, 3731, 
    3642, 3943, 3623, 3645, 3637, 3950, 
    6074, 3644, 3864, 3940, 3865, 6117, 
    3869, 3948, 3941, 6068, 3951, 5812, 
    3949, 6097, 8612, 6073, 8464, 8469, 
    10025, 8724, 8445, 8811, 8441, 8446, 
    8440, 8593, 8595, 8444, 8471, 8594, 
    8457, 8828, 8466, 8831, 8276, 8365, 
    8585
  ) 
GROUP BY 
  p.product_id

Query time 0.00105

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "2.31"
    },
    "grouping_operation": {
      "using_temporary_table": true,
      "using_filesort": false,
      "nested_loop": [
        {
          "table": {
            "table_name": "dp",
            "access_type": "ALL",
            "possible_keys": [
              "thread_id",
              "thread_id_2"
            ],
            "rows_examined_per_scan": 13,
            "rows_produced_per_join": 1,
            "filtered": "10.00",
            "cost_info": {
              "read_cost": "1.70",
              "eval_cost": "0.13",
              "prefix_cost": "1.83",
              "data_read_per_join": "582"
            },
            "used_columns": [
              "post_id",
              "thread_id",
              "status"
            ],
            "attached_condition": "(`softwarepirmam_hewadelivard_cscart_4`.`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": [
              "softwarepirmam_hewadelivard_cscart_4.dp.thread_id"
            ],
            "rows_examined_per_scan": 1,
            "rows_produced_per_join": 0,
            "filtered": "5.00",
            "cost_info": {
              "read_cost": "0.33",
              "eval_cost": "0.01",
              "prefix_cost": "2.28",
              "data_read_per_join": "1"
            },
            "used_columns": [
              "thread_id",
              "object_id",
              "object_type"
            ],
            "attached_condition": "((`softwarepirmam_hewadelivard_cscart_4`.`cscart_discussion`.`object_id` in (5696,5621,5892,5719,5706,5720,5703,4894,4962,5691,5687,3733,4915,3589,4976,3811,6093,6194,4935,3734,3641,5700,6064,3798,4931,3730,3638,3729,3851,6204,3942,3732,3448,3728,3852,3850,3731,3642,3943,3623,3645,3637,3950,6074,3644,3864,3940,3865,6117,3869,3948,3941,6068,3951,5812,3949,6097,8612,6073,8464,8469,10025,8724,8445,8811,8441,8446,8440,8593,8595,8444,8471,8594,8457,8828,8466,8831,8276,8365,8585)) and (`softwarepirmam_hewadelivard_cscart_4`.`cscart_discussion`.`object_type` = 'P'))"
          }
        },
        {
          "table": {
            "table_name": "p",
            "access_type": "eq_ref",
            "possible_keys": [
              "PRIMARY"
            ],
            "key": "PRIMARY",
            "used_key_parts": [
              "product_id"
            ],
            "key_length": "3",
            "ref": [
              "softwarepirmam_hewadelivard_cscart_4.cscart_discussion.object_id"
            ],
            "rows_examined_per_scan": 1,
            "rows_produced_per_join": 0,
            "filtered": "100.00",
            "using_index": true,
            "cost_info": {
              "read_cost": "0.02",
              "eval_cost": "0.01",
              "prefix_cost": "2.31",
              "data_read_per_join": "294"
            },
            "used_columns": [
              "product_id"
            ]
          }
        }
      ]
    }
  }
}