问题
According to the reference documentation the READ ONLY transaction flag is useful other than allowing DEFERRABLE transactions?
SET SESSION CHARACTERISTICS AS TRANSACTION READ ONLY;
The DEFERRABLE transaction property has no effect unless the transaction is also SERIALIZABLE and READ ONLY. When all three of these properties are selected for a transaction, the transaction may block when first acquiring its snapshot, after which it is able to run without the normal overhead of a SERIALIZABLE transaction and without any risk of contributing to or being canceled by a serialization failure. This mode is well suited for long-running reports or backups.
Does the database engine runs other optimizations for read-only transactions?
回答1:
To sum up the comments from Nick Barnes and Craig Ringer in the question comments:
- The READ_ONLY flag does not necessarily provide any optimization
- The main benefit of setting the READ_ONLY flag is to ensure that no tuple is going to be modified
回答2:
Actually, it does. Let me just cite source code comment here:
/*
* 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.
*/
来源：https://stackoverflow.com/questions/33332706/does-postgresql-run-some-performance-optimizations-for-read-only-transactions