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 (
    10991, 10992, 11025, 11024, 11081, 11080, 
    11082, 11086, 11085, 11137, 11136, 
    11152, 11153, 11236, 11237, 11300, 
    11301, 11306, 11307, 11337, 11338, 
    11348, 11349, 11352, 11355, 11360, 
    11361, 11362, 11363, 11364, 11365, 
    11388, 11389, 11390, 11391, 11399, 
    11400, 11411, 11412, 11413, 11416, 
    11415, 11414, 11417, 11430, 11431, 
    11439, 11441, 11445, 11446, 11447, 
    11448, 11453, 11454, 11455, 11458, 
    11460, 11462, 11463, 11472, 11474, 
    11475, 11477, 11478, 11480, 11479, 
    11481, 11508, 11509, 11520, 11521, 
    11522, 11523, 11530, 11532, 11531, 
    11533, 11534, 11535, 11536, 11598, 
    11599, 11600, 11633, 11634, 11635, 
    11636
  ) 
GROUP BY 
  p.product_id

Query time 0.00078

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 (10991,10992,11025,11024,11081,11080,11082,11086,11085,11137,11136,11152,11153,11236,11237,11300,11301,11306,11307,11337,11338,11348,11349,11352,11355,11360,11361,11362,11363,11364,11365,11388,11389,11390,11391,11399,11400,11411,11412,11413,11416,11415,11414,11417,11430,11431,11439,11441,11445,11446,11447,11448,11453,11454,11455,11458,11460,11462,11463,11472,11474,11475,11477,11478,11480,11479,11481,11508,11509,11520,11521,11522,11523,11530,11532,11531,11533,11534,11535,11536,11598,11599,11600,11633,11634,11635,11636)) 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"
            ]
          }
        }
      ]
    }
  }
}