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 (
    1952, 1939, 1126, 1129, 1125, 2007, 2076, 
    2352, 1578, 1127, 2030, 2269, 2289, 
    1173, 2167, 2128, 1574, 1968, 1969, 
    1579, 1575, 1128, 1081, 2192, 1082
  ) 
GROUP BY 
  p.product_id

Query time 0.00037

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "1.47"
    },
    "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": 5,
            "rows_produced_per_join": 1,
            "filtered": "20.00",
            "cost_info": {
              "read_cost": "1.01",
              "eval_cost": "0.10",
              "prefix_cost": "1.11",
              "data_read_per_join": "448"
            },
            "used_columns": [
              "post_id",
              "thread_id",
              "status"
            ],
            "attached_condition": "(`cscartdevel`.`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": [
              "cscartdevel.dp.thread_id"
            ],
            "rows_examined_per_scan": 1,
            "rows_produced_per_join": 0,
            "filtered": "5.00",
            "cost_info": {
              "read_cost": "0.25",
              "eval_cost": "0.01",
              "prefix_cost": "1.46",
              "data_read_per_join": "1"
            },
            "used_columns": [
              "thread_id",
              "object_id",
              "object_type"
            ],
            "attached_condition": "((`cscartdevel`.`cscart_discussion`.`object_id` in (1952,1939,1126,1129,1125,2007,2076,2352,1578,1127,2030,2269,2289,1173,2167,2128,1574,1968,1969,1579,1575,1128,1081,2192,1082)) and (`cscartdevel`.`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": [
              "cscartdevel.cscart_discussion.object_id"
            ],
            "rows_examined_per_scan": 1,
            "rows_produced_per_join": 0,
            "filtered": "100.00",
            "using_index": true,
            "cost_info": {
              "read_cost": "0.01",
              "eval_cost": "0.01",
              "prefix_cost": "1.47",
              "data_read_per_join": "234"
            },
            "used_columns": [
              "product_id"
            ]
          }
        }
      ]
    }
  }
}