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 (
    1820, 1897, 1760, 1807, 1847, 2363, 1816, 
    1819, 1849, 1802, 1801, 1880, 1855, 
    1808, 1891, 1892, 1821, 2164, 1786, 
    1805, 1822, 2170, 1861, 1856, 1846, 
    1885, 1873, 1824, 1869, 2361, 1825, 
    1826, 1749, 1752, 1827, 1870, 1803, 
    1828, 1754, 1770, 1879, 1874, 1829, 
    1864, 1904, 1842, 1781, 1894, 2153, 
    1886, 1887, 1746, 1753, 1875, 1830, 
    1863, 1852, 1906, 1757, 1895, 1800, 
    1848, 1977, 1832, 1765, 1768, 1769, 
    1835, 1871, 1838, 1766, 1903, 1773, 
    1762, 1763
  ) 
GROUP BY 
  p.product_id

Query time 0.00046

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": "5.25",
            "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 (1820,1897,1760,1807,1847,2363,1816,1819,1849,1802,1801,1880,1855,1808,1891,1892,1821,2164,1786,1805,1822,2170,1861,1856,1846,1885,1873,1824,1869,2361,1825,1826,1749,1752,1827,1870,1803,1828,1754,1770,1879,1874,1829,1864,1904,1842,1781,1894,2153,1886,1887,1746,1753,1875,1830,1863,1852,1906,1757,1895,1800,1848,1977,1832,1765,1768,1769,1835,1871,1838,1766,1903,1773,1762,1763)) 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"
            ]
          }
        }
      ]
    }
  }
}