SELECT 
  p.product_id, 
  ifnull(
    count(dp.post_id), 
    0
  ) as discussion_amount_posts 
FROM 
  jmj_discussion 
  INNER JOIN jmj_products as p ON (
    jmj_discussion.object_id = p.product_id
  ) 
  INNER JOIN jmj_discussion_posts as dp ON (
    jmj_discussion.thread_id = dp.thread_id 
    AND jmj_discussion.object_type = 'P'
  ) 
WHERE 
  dp.status = 'A' 
  and p.product_id in (
    1678, 1539, 1536, 915, 914, 913, 912, 
    911, 797, 796
  ) 
GROUP BY 
  p.product_id

Query time 0.00062

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "10.55"
    },
    "grouping_operation": {
      "using_temporary_table": true,
      "using_filesort": true,
      "cost_info": {
        "sort_cost": "0.13"
      },
      "nested_loop": [
        {
          "table": {
            "table_name": "dp",
            "access_type": "ALL",
            "possible_keys": [
              "thread_id",
              "thread_id_2"
            ],
            "rows_examined_per_scan": 25,
            "rows_produced_per_join": 2,
            "filtered": "10.00",
            "cost_info": {
              "read_cost": "6.78",
              "eval_cost": "0.50",
              "prefix_cost": "7.28",
              "data_read_per_join": "1K"
            },
            "used_columns": [
              "post_id",
              "thread_id",
              "status"
            ],
            "attached_condition": "(`jmj_test`.`dp`.`status` = 'A')"
          }
        },
        {
          "table": {
            "table_name": "jmj_discussion",
            "access_type": "eq_ref",
            "possible_keys": [
              "PRIMARY",
              "object_id"
            ],
            "key": "PRIMARY",
            "used_key_parts": [
              "thread_id"
            ],
            "key_length": "3",
            "ref": [
              "jmj_test.dp.thread_id"
            ],
            "rows_examined_per_scan": 1,
            "rows_produced_per_join": 0,
            "filtered": "5.00",
            "cost_info": {
              "read_cost": "2.50",
              "eval_cost": "0.03",
              "prefix_cost": "10.28",
              "data_read_per_join": "3"
            },
            "used_columns": [
              "thread_id",
              "object_id",
              "object_type"
            ],
            "attached_condition": "((`jmj_test`.`jmj_discussion`.`object_id` in (1678,1539,1536,915,914,913,912,911,797,796)) and (`jmj_test`.`jmj_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": [
              "jmj_test.jmj_discussion.object_id"
            ],
            "rows_examined_per_scan": 1,
            "rows_produced_per_join": 0,
            "filtered": "100.00",
            "using_index": true,
            "cost_info": {
              "read_cost": "0.13",
              "eval_cost": "0.03",
              "prefix_cost": "10.43",
              "data_read_per_join": "606"
            },
            "used_columns": [
              "product_id"
            ]
          }
        }
      ]
    }
  }
}