使用简单的AUTO_INCREMENT并忽略存储:

Use a simple AUTO_INCREMENT and ignore the storage:

• 如果您将规模扩大到一百万个用户，每个用户拥有1000个帐户，您将 需要大约3MB的额外存储空间.
• 单字段主索引(主要在实际存储中排序)的性能优势将比那一点存储价值更多.

• If you scale to a million users, each having 1000 accounts, you will need approx. 3MB of additional storage.
• The performance benefit of a single-field primary index (mostly sorted in real storage!) will be worth much more, than that bit of storage