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 (
    1914, 1915, 1761, 2203, 1789, 1811, 1791, 
    1889, 1836, 1799, 1845, 1792, 1905, 
    1806, 1837, 2282, 2002, 1940, 2295, 
    2283, 1970, 2387, 2014, 2355, 1996, 
    2286, 2285, 2310, 1997, 1956, 1971, 
    2316, 1955, 2334, 2108, 2360, 2287, 
    2288, 2009, 1950, 2109, 2107, 2221, 
    2023, 2123, 1982, 1944, 2031, 2032, 
    1920, 2033, 2034, 2165, 2223, 2190, 
    1758, 2365, 2067, 2294, 2070, 2069, 
    1835, 1871, 2241, 2351, 2091, 1959, 
    1981, 2137, 2340, 1745, 2209, 2063, 
    2370, 2377, 1916, 2381, 2392, 1838, 
    1766
  ) 
GROUP BY 
  p.product_id

Query time 0.00039

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 (1914,1915,1761,2203,1789,1811,1791,1889,1836,1799,1845,1792,1905,1806,1837,2282,2002,1940,2295,2283,1970,2387,2014,2355,1996,2286,2285,2310,1997,1956,1971,2316,1955,2334,2108,2360,2287,2288,2009,1950,2109,2107,2221,2023,2123,1982,1944,2031,2032,1920,2033,2034,2165,2223,2190,1758,2365,2067,2294,2070,2069,1835,1871,2241,2351,2091,1959,1981,2137,2340,1745,2209,2063,2370,2377,1916,2381,2392,1838,1766)) 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"
            ]
          }
        }
      ]
    }
  }
}