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 (
    1753, 2332, 1875, 1830, 1863, 2036, 2057, 
    2082, 2138, 2317, 2277, 1925, 1831, 
    1775, 1932, 2364, 1840, 2163, 2173, 
    2175, 2177, 2176, 2169, 2191, 2168, 
    1983, 2302, 1899, 2240, 1910, 1872, 
    1850, 1883, 1777, 1783, 1852, 1906, 
    1810, 1851, 2154, 1884, 2156, 1907, 
    1909, 1908, 1757, 1895, 1800, 2230, 
    1848, 1977, 1832, 2155, 1862, 2161, 
    1978, 1902, 1979, 2162, 1890, 1901, 
    1854, 1833, 1844, 1804, 1756, 1834, 
    1788, 1809, 1778, 1865, 2160, 1866, 
    2210, 1765, 1768, 1769, 2180, 2181, 
    2179
  ) 
GROUP BY 
  p.product_id

Query time 0.00044

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 (1753,2332,1875,1830,1863,2036,2057,2082,2138,2317,2277,1925,1831,1775,1932,2364,1840,2163,2173,2175,2177,2176,2169,2191,2168,1983,2302,1899,2240,1910,1872,1850,1883,1777,1783,1852,1906,1810,1851,2154,1884,2156,1907,1909,1908,1757,1895,1800,2230,1848,1977,1832,2155,1862,2161,1978,1902,1979,2162,1890,1901,1854,1833,1844,1804,1756,1834,1788,1809,1778,1865,2160,1866,2210,1765,1768,1769,2180,2181,2179)) 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"
            ]
          }
        }
      ]
    }
  }
}