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 (
    2942, 3088, 3090, 3091, 3043, 3092, 3093, 
    3094, 3095, 3096, 3097, 3098, 3099, 
    3100, 3101, 3102, 3104, 3105, 3106, 
    3107, 3108, 3109, 3110, 3111, 2946, 
    3130, 3131, 3132, 3133, 2951, 3135, 
    3136, 3137, 3033, 3139, 3140, 3141, 
    3142, 3028, 3162, 3163, 3164, 3165, 
    3038, 3167, 3168, 3169, 3170, 3071, 
    3172, 3173, 3067, 3175, 3176, 3177, 
    3178, 3179, 3180, 3181, 2955, 4667, 
    4668, 4669, 2959, 5806, 5807, 5808, 
    2963, 5986, 5987, 5988, 5989, 5990, 
    5991, 5992, 5993, 2972, 5999, 6000, 
    6001, 2980, 6003, 6004, 6005, 2976, 
    6007, 6008, 6009, 2984, 6011, 6012, 
    6013
  ) 
GROUP BY 
  p.product_id

Query time 0.00079

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 (2942,3088,3090,3091,3043,3092,3093,3094,3095,3096,3097,3098,3099,3100,3101,3102,3104,3105,3106,3107,3108,3109,3110,3111,2946,3130,3131,3132,3133,2951,3135,3136,3137,3033,3139,3140,3141,3142,3028,3162,3163,3164,3165,3038,3167,3168,3169,3170,3071,3172,3173,3067,3175,3176,3177,3178,3179,3180,3181,2955,4667,4668,4669,2959,5806,5807,5808,2963,5986,5987,5988,5989,5990,5991,5992,5993,2972,5999,6000,6001,2980,6003,6004,6005,2976,6007,6008,6009,2984,6011,6012,6013)) 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"
            ]
          }
        }
      ]
    }
  }
}