SELECT 
  rf_rent 
FROM 
  netting_categories 
WHERE 
  category_id IN (315, 310) 
  AND rf_rent = 'Y'

Query time 0.00054

JSON explain

{
  "query_block": {
    "select_id": 1,
    "nested_loop": [
      {
        "table": {
          "table_name": "netting_categories",
          "access_type": "range",
          "possible_keys": ["PRIMARY", "p_category_id"],
          "key": "PRIMARY",
          "key_length": "3",
          "used_key_parts": ["category_id"],
          "rows": 2,
          "filtered": 100,
          "index_condition": "netting_categories.category_id in (315,310)",
          "attached_condition": "netting_categories.rf_rent = 'Y'"
        }
      }
    ]
  }
}