SELECT 
  option_id, 
  variant_id, 
  modifier, 
  modifier_type, 
  position 
FROM 
  cscart_nested_option_variant_modifiers 
WHERE 
  product_id = 12684

Query time 0.00052

JSON explain

{
  "query_block": {
    "select_id": 1,
    "cost_info": {
      "query_cost": "1.35"
    },
    "table": {
      "table_name": "cscart_nested_option_variant_modifiers",
      "access_type": "ref",
      "possible_keys": [
        "uniq_variant_modifier",
        "idx_product"
      ],
      "key": "uniq_variant_modifier",
      "used_key_parts": [
        "product_id"
      ],
      "key_length": "4",
      "ref": [
        "const"
      ],
      "rows_examined_per_scan": 6,
      "rows_produced_per_join": 6,
      "filtered": "100.00",
      "cost_info": {
        "read_cost": "0.75",
        "eval_cost": "0.60",
        "prefix_cost": "1.35",
        "data_read_per_join": "192"
      },
      "used_columns": [
        "product_id",
        "option_id",
        "variant_id",
        "modifier",
        "modifier_type",
        "position"
      ]
    }
  }
}