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 (
    1893, 1857, 2147, 1790, 1877, 1858, 1876, 
    1841, 1796, 2329, 2330, 1860, 2148, 
    2152, 1868, 1881, 1888, 2303, 2016, 
    2319, 1999, 2019, 2321, 2322, 2018, 
    1928, 2017, 1998, 2020, 2071, 2320, 
    2333, 2080, 2325, 2113, 2324, 2327, 
    2326, 2347, 2323, 2135, 2300, 2206, 
    2193, 2079, 2078, 1921, 2296, 2012, 
    2013, 2280, 2335, 2380, 2035, 1941, 
    2052, 2055, 1960, 1926, 1990, 2341, 
    2053, 1937, 2353, 2072, 2074, 2073, 
    2312, 2313, 2084, 2292, 2304, 1821, 
    2164, 1786, 1805, 1822, 2170, 2189, 
    2188
  ) 
GROUP BY 
  p.product_id

Query time 0.00045

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": "5.24",
            "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 (1893,1857,2147,1790,1877,1858,1876,1841,1796,2329,2330,1860,2148,2152,1868,1881,1888,2303,2016,2319,1999,2019,2321,2322,2018,1928,2017,1998,2020,2071,2320,2333,2080,2325,2113,2324,2327,2326,2347,2323,2135,2300,2206,2193,2079,2078,1921,2296,2012,2013,2280,2335,2380,2035,1941,2052,2055,1960,1926,1990,2341,2053,1937,2353,2072,2074,2073,2312,2313,2084,2292,2304,1821,2164,1786,1805,1822,2170,2189,2188)) 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": "1.48",
              "data_read_per_join": "245"
            },
            "used_columns": [
              "product_id"
            ]
          }
        }
      ]
    }
  }
}