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 (
    969, 970, 1020, 1021, 632, 633, 1013, 
    1014, 1016, 1017, 613, 614, 615, 616, 
    617, 618, 619, 620, 622, 636, 637, 638, 
    639, 972, 973, 974, 975, 980, 981, 982, 
    983, 984, 985, 986, 987, 988, 990, 991, 
    992, 993, 994, 995, 996, 997, 998, 999, 
    1000, 1001, 1002, 1003, 1004, 1005, 
    1006, 1007, 1008, 1009, 1010, 1011, 
    1030, 621, 1029
  ) 
GROUP BY 
  p.product_id

Query time 0.00059

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "5.71"
    },
    "grouping_operation": {
      "using_temporary_table": true,
      "using_filesort": true,
      "cost_info": {
        "sort_cost": "0.17"
      },
      "nested_loop": [
        {
          "table": {
            "table_name": "dp",
            "access_type": "ALL",
            "possible_keys": [
              "thread_id",
              "thread_id_2"
            ],
            "rows_examined_per_scan": 10,
            "rows_produced_per_join": 1,
            "filtered": "10.00",
            "cost_info": {
              "read_cost": "3.94",
              "eval_cost": "0.20",
              "prefix_cost": "4.14",
              "data_read_per_join": "448"
            },
            "used_columns": [
              "post_id",
              "thread_id",
              "status"
            ],
            "attached_condition": "(`cscart`.`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": [
              "cscart.dp.thread_id"
            ],
            "rows_examined_per_scan": 1,
            "rows_produced_per_join": 0,
            "filtered": "16.80",
            "cost_info": {
              "read_cost": "1.00",
              "eval_cost": "0.03",
              "prefix_cost": "5.34",
              "data_read_per_join": "4"
            },
            "used_columns": [
              "thread_id",
              "object_id",
              "object_type"
            ],
            "attached_condition": "((`cscart`.`cscart_discussion`.`object_id` in (969,970,1020,1021,632,633,1013,1014,1016,1017,613,614,615,616,617,618,619,620,622,636,637,638,639,972,973,974,975,980,981,982,983,984,985,986,987,988,990,991,992,993,994,995,996,997,998,999,1000,1001,1002,1003,1004,1005,1006,1007,1008,1009,1010,1011,1030,621,1029)) and (`cscart`.`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": [
              "cscart.cscart_discussion.object_id"
            ],
            "rows_examined_per_scan": 1,
            "rows_produced_per_join": 0,
            "filtered": "100.00",
            "using_index": true,
            "cost_info": {
              "read_cost": "0.17",
              "eval_cost": "0.03",
              "prefix_cost": "5.55",
              "data_read_per_join": "1K"
            },
            "used_columns": [
              "product_id"
            ]
          }
        }
      ]
    }
  }
}