Does PostgreSQL run some performance optimizations for read-only transactions
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
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. */