PostgreSQL是否为只读事务运行一些性能优化

Question

根据参考文档,除了允许DEFERRABLE事务之外,READ ONLY事务标志是有用的吗？

SET SESSION CHARACTERISTICS AS TRANSACTION READ ONLY;

除非事务也是SERIALIZABLE和READ ONLY,否则DEFERRABLE事务属性不起作用.当为事务选择所有这三个属性时,事务可能会在第一次获取其快照时阻塞,之后它可以在没有SERIALIZABLE事务的正常开销的情况下运行,并且没有任何可能导致序列化或被序列化取消的风险失败.此模式非常适合长时间运行的报告或备份.

数据库引擎是否为只读事务运行其他优化？

Answer 1

总结Nick Barnes和Craig Ringer在评论中的评论:

1. READ_ONLY标志不一定提供任何优化
2. 设置READ_ONLY标志的主要好处是确保不会修改元组

Answer 2

事实上，确实如此。让我在这里引用源代码注释：

/*
 * Check if we have just become "RO-safe". If we have, immediately release
 * all locks as they're not needed anymore. This also resets
 * MySerializableXact, so that subsequent calls to this function can exit
 * quickly.
 *
 * A transaction is flagged as RO_SAFE if all concurrent R/W transactions
 * commit without having conflicts out to an earlier snapshot, thus
 * ensuring that no conflicts are possible for this transaction.
 */