SELECT 
  p.product_id, 
  ifnull(
    count(dp.post_id), 
    0
  ) as discussion_amount_posts 
FROM 
  q_discussion 
  INNER JOIN q_products as p ON (
    q_discussion.object_id = p.product_id
  ) 
  INNER JOIN q_discussion_posts as dp ON (
    q_discussion.thread_id = dp.thread_id 
    AND q_discussion.object_type = 'P'
  ) 
WHERE 
  dp.status = 'A' 
  and p.product_id in (
    32484, 32485, 32486, 32487, 32488, 32489, 
    32490, 32491, 32492, 32493, 32494, 
    32495, 32496, 32497, 32499, 32500, 
    32501, 32502, 32503, 32504, 32505, 
    32506, 32507, 32508, 32509, 32510, 
    32511, 32512, 32513, 32514, 32515, 
    32516
  ) 
GROUP BY 
  p.product_id

Query time 0.00066

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "21.61"
    },
    "grouping_operation": {
      "using_temporary_table": true,
      "using_filesort": true,
      "cost_info": {
        "sort_cost": "0.29"
      },
      "nested_loop": [
        {
          "table": {
            "table_name": "dp",
            "access_type": "ALL",
            "possible_keys": [
              "thread_id",
              "thread_id_2"
            ],
            "rows_examined_per_scan": 57,
            "rows_produced_per_join": 5,
            "filtered": "10.00",
            "cost_info": {
              "read_cost": "13.01",
              "eval_cost": "1.14",
              "prefix_cost": "14.15",
              "data_read_per_join": "2K"
            },
            "used_columns": [
              "post_id",
              "thread_id",
              "status"
            ],
            "attached_condition": "(`portal`.`dp`.`status` = 'A')"
          }
        },
        {
          "table": {
            "table_name": "q_discussion",
            "access_type": "eq_ref",
            "possible_keys": [
              "PRIMARY",
              "object_id"
            ],
            "key": "PRIMARY",
            "used_key_parts": [
              "thread_id"
            ],
            "key_length": "3",
            "ref": [
              "portal.dp.thread_id"
            ],
            "rows_examined_per_scan": 1,
            "rows_produced_per_join": 0,
            "filtered": "5.00",
            "cost_info": {
              "read_cost": "5.70",
              "eval_cost": "0.06",
              "prefix_cost": "20.99",
              "data_read_per_join": "4"
            },
            "used_columns": [
              "thread_id",
              "object_id",
              "object_type"
            ],
            "attached_condition": "((`portal`.`q_discussion`.`object_id` in (32484,32485,32486,32487,32488,32489,32490,32491,32492,32493,32494,32495,32496,32497,32499,32500,32501,32502,32503,32504,32505,32506,32507,32508,32509,32510,32511,32512,32513,32514,32515,32516)) and (`portal`.`q_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": [
              "portal.q_discussion.object_id"
            ],
            "rows_examined_per_scan": 1,
            "rows_produced_per_join": 0,
            "filtered": "100.00",
            "using_index": true,
            "cost_info": {
              "read_cost": "0.29",
              "eval_cost": "0.06",
              "prefix_cost": "21.33",
              "data_read_per_join": "1K"
            },
            "used_columns": [
              "product_id"
            ]
          }
        }
      ]
    }
  }
}