PostgreSQL是否针对只读事务运行一些性能优化

Question

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?

Answer 1

To sum up the comments from Nick Barnes and Craig Ringer in the question comments:

1. The READ_ONLY flag does not necessarily provide any optimization
2. The main benefit of setting the READ_ONLY flag is to ensure that no tuple is going to be modified

Answer 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.
 */