SELECT 
  feature_id 
FROM 
  cscart_product_features 
WHERE 
  feature_id IN (3, 1, 6) 
  AND feature_type = 'E'

Query time 0.00018

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "2.11"
    },
    "table": {
      "table_name": "cscart_product_features",
      "access_type": "range",
      "possible_keys": [
        "PRIMARY"
      ],
      "key": "PRIMARY",
      "used_key_parts": [
        "feature_id"
      ],
      "key_length": "3",
      "rows_examined_per_scan": 3,
      "rows_produced_per_join": 0,
      "filtered": "20.00",
      "index_condition": "(`betaatif_store`.`cscart_product_features`.`feature_id` in (3,1,6))",
      "cost_info": {
        "read_cost": "2.05",
        "eval_cost": "0.06",
        "prefix_cost": "2.11",
        "data_read_per_join": "264"
      },
      "used_columns": [
        "feature_id",
        "feature_type"
      ],
      "attached_condition": "(`betaatif_store`.`cscart_product_features`.`feature_type` = 'E')"
    }
  }
}