SELECT 
  cscart_ab__mb_motivation_item_objects.motivation_item_id, 
  cscart_ab__mb_motivation_item_objects.object_type, 
  GROUP_CONCAT(
    cscart_ab__mb_motivation_item_objects.object_id
  ) as object_ids 
FROM 
  cscart_ab__mb_motivation_item_objects 
WHERE 
  1 
  AND cscart_ab__mb_motivation_item_objects.motivation_item_id IN (13, 14, 15) 
  AND cscart_ab__mb_motivation_item_objects.object_type IN ('C', 'D', 'P') 
GROUP BY 
  cscart_ab__mb_motivation_item_objects.motivation_item_id, 
  cscart_ab__mb_motivation_item_objects.object_type

Query time 0.00038

JSON explain

{
  "query_block": {
    "select_id": 1,
    "table": {
      "table_name": "cscart_ab__mb_motivation_item_objects",
      "access_type": "range",
      "possible_keys": ["PRIMARY", "mid_otype"],
      "key": "mid_otype",
      "key_length": "6",
      "used_key_parts": ["motivation_item_id", "object_type"],
      "rows": 9,
      "filtered": 100,
      "attached_condition": "cscart_ab__mb_motivation_item_objects.motivation_item_id in (13,14,15) and cscart_ab__mb_motivation_item_objects.object_type in ('C','D','P')",
      "using_index": true
    }
  }
}