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 (
    16150, 15902, 16716, 16813, 16812, 16849, 
    17017, 16845, 16932, 16888, 16889, 
    15798, 16861, 16862, 16943, 15835, 
    15836, 15839, 15834, 15837, 16881, 
    15844, 16929, 16938, 16937, 16152, 
    16153, 16947, 15912, 15913, 15797, 
    13030, 13029, 15832, 16801, 16798, 
    16800, 16946, 16827, 16826, 16871, 
    16858, 15845, 16345, 15910, 16941, 
    16940, 16939
  ) 
GROUP BY 
  p.product_id

Query time 0.00115

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "1.69"
    },
    "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": 8,
            "rows_produced_per_join": 1,
            "filtered": "12.50",
            "cost_info": {
              "read_cost": "1.22",
              "eval_cost": "0.10",
              "prefix_cost": "1.32",
              "data_read_per_join": "448"
            },
            "used_columns": [
              "post_id",
              "thread_id",
              "status"
            ],
            "attached_condition": "(`dev_yourmart_co_bw`.`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": [
              "dev_yourmart_co_bw.dp.thread_id"
            ],
            "rows_examined_per_scan": 1,
            "rows_produced_per_join": 0,
            "filtered": "5.00",
            "cost_info": {
              "read_cost": "0.25",
              "eval_cost": "0.01",
              "prefix_cost": "1.67",
              "data_read_per_join": "1"
            },
            "used_columns": [
              "thread_id",
              "object_id",
              "object_type"
            ],
            "attached_condition": "((`dev_yourmart_co_bw`.`cscart_discussion`.`object_id` in (16150,15902,16716,16813,16812,16849,17017,16845,16932,16888,16889,15798,16861,16862,16943,15835,15836,15839,15834,15837,16881,15844,16929,16938,16937,16152,16153,16947,15912,15913,15797,13030,13029,15832,16801,16798,16800,16946,16827,16826,16871,16858,15845,16345,15910,16941,16940,16939)) and (`dev_yourmart_co_bw`.`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": [
              "dev_yourmart_co_bw.cscart_discussion.object_id"
            ],
            "rows_examined_per_scan": 1,
            "rows_produced_per_join": 0,
            "filtered": "100.00",
            "using_index": true,
            "cost_info": {
              "read_cost": "0.01",
              "eval_cost": "0.01",
              "prefix_cost": "1.69",
              "data_read_per_join": "266"
            },
            "used_columns": [
              "product_id"
            ]
          }
        }
      ]
    }
  }
}