SELECT 
  jmj_ab__mb_motivation_item_objects.motivation_item_id, 
  jmj_ab__mb_motivation_item_objects.object_type, 
  GROUP_CONCAT(
    jmj_ab__mb_motivation_item_objects.object_id
  ) as object_ids 
FROM 
  jmj_ab__mb_motivation_item_objects 
WHERE 
  1 
  AND jmj_ab__mb_motivation_item_objects.motivation_item_id IN (1, 2, 3, 4) 
  AND jmj_ab__mb_motivation_item_objects.object_type IN ('C', 'D', 'P') 
GROUP BY 
  jmj_ab__mb_motivation_item_objects.motivation_item_id, 
  jmj_ab__mb_motivation_item_objects.object_type

Query time 0.00232

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "3.40"
    },
    "grouping_operation": {
      "using_filesort": false,
      "table": {
        "table_name": "jmj_ab__mb_motivation_item_objects",
        "access_type": "index",
        "possible_keys": [
          "PRIMARY",
          "mid_otype"
        ],
        "key": "mid_otype",
        "used_key_parts": [
          "motivation_item_id",
          "object_type"
        ],
        "key_length": "6",
        "rows_examined_per_scan": 12,
        "rows_produced_per_join": 3,
        "filtered": "30.00",
        "using_index": true,
        "cost_info": {
          "read_cost": "2.68",
          "eval_cost": "0.72",
          "prefix_cost": "3.40",
          "data_read_per_join": "57"
        },
        "used_columns": [
          "motivation_item_id",
          "object_id",
          "object_type"
        ],
        "attached_condition": "((`jmj_test`.`jmj_ab__mb_motivation_item_objects`.`motivation_item_id` in (1,2,3,4)) and (`jmj_test`.`jmj_ab__mb_motivation_item_objects`.`object_type` in ('C','D','P')))"
      }
    }
  }
}