SELECT 
  product_id, 
  amount, 
  all_variants 
FROM 
  cscart_product_bundle_product_links 
WHERE 
  bundle_id = 3

Query time 0.00031

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "2.60"
    },
    "table": {
      "table_name": "cscart_product_bundle_product_links",
      "access_type": "ref",
      "possible_keys": [
        "bundle_id"
      ],
      "key": "bundle_id",
      "used_key_parts": [
        "bundle_id"
      ],
      "key_length": "4",
      "ref": [
        "const"
      ],
      "rows_examined_per_scan": 3,
      "rows_produced_per_join": 3,
      "filtered": "100.00",
      "cost_info": {
        "read_cost": "2.00",
        "eval_cost": "0.60",
        "prefix_cost": "2.60",
        "data_read_per_join": "72"
      },
      "used_columns": [
        "bundle_id",
        "product_id",
        "amount",
        "all_variants"
      ]
    }
  }
}