SELECT 
  object_id 
FROM 
  cscart_images_links 
WHERE 
  object_type = 'abt__ut2/menu-with-icon'

Query time 0.09991

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "33513.16"
    },
    "table": {
      "table_name": "cscart_images_links",
      "access_type": "index",
      "possible_keys": [
        "object_id"
      ],
      "key": "object_id",
      "used_key_parts": [
        "object_id",
        "object_type",
        "type"
      ],
      "key_length": "81",
      "rows_examined_per_scan": 329501,
      "rows_produced_per_join": 32950,
      "filtered": "10.00",
      "using_index": true,
      "cost_info": {
        "read_cost": "30218.15",
        "eval_cost": "3295.01",
        "prefix_cost": "33513.16",
        "data_read_per_join": "3M"
      },
      "used_columns": [
        "object_id",
        "object_type"
      ],
      "attached_condition": "(`s2cart`.`cscart_images_links`.`object_type` = 'abt__ut2/menu-with-icon')"
    }
  }
}