[英]Does PostgreSQL run some performance optimizations for read-only transactions
根据参考文档 ,除了允许DEFERRABLE事务以外,READ ONLY事务标志还有用。
SET SESSION CHARACTERISTICS AS TRANSACTION READ ONLY;
除非事务也是可序列化的并且只能读取,否则DEFERRABLE事务属性无效。 当为一个事务选择了所有这三个属性时,该事务在首次获取其快照时可能会阻塞,此后它可以在没有SERIALIZABLE事务的正常开销的情况下运行,并且没有任何有助于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。
数据库引擎是否对只读事务运行其他优化?
总结一下尼克·巴恩斯和克雷格·林格在问题评论中的评论:
实际上,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.
*/
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