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 (
    11508, 11147, 11306, 11446, 11472, 11531, 
    11086, 11126, 11166, 11291, 11337, 
    11348, 11411, 11477, 11167, 11256, 
    11309, 11430, 11479, 11600, 11362, 
    11414, 11520, 11633, 11128, 11390, 
    11535, 11199, 11399, 11530, 11236, 
    11289, 11462, 11136, 11193, 11352, 
    11458, 11267, 11293, 11308, 11388, 
    11143, 11152, 11231, 11238, 11453, 
    11439, 11202, 11274, 11360, 11522, 
    11198, 11272, 11300, 11500, 11168, 
    11217, 11233, 11287, 11364, 11218, 
    11226, 11232, 11271, 11164, 11288, 
    11526, 11024, 11220, 11080, 11204, 
    11200, 11270, 10991, 11003, 11211, 
    11216, 11240, 11084, 11210
  ) 
GROUP BY 
  p.product_id

Query time 0.00235

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 (11508,11147,11306,11446,11472,11531,11086,11126,11166,11291,11337,11348,11411,11477,11167,11256,11309,11430,11479,11600,11362,11414,11520,11633,11128,11390,11535,11199,11399,11530,11236,11289,11462,11136,11193,11352,11458,11267,11293,11308,11388,11143,11152,11231,11238,11453,11439,11202,11274,11360,11522,11198,11272,11300,11500,11168,11217,11233,11287,11364,11218,11226,11232,11271,11164,11288,11526,11024,11220,11080,11204,11200,11270,10991,11003,11211,11216,11240,11084,11210)) 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"
            ]
          }
        }
      ]
    }
  }
}