Does PostgreSQL run some performance optimizations for read-only transactions

Question

According to the reference documentation the READ ONLY transaction flag is useful other than allowing DEFERRABLE transactions?

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Answer 1

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.
 */

Answer 2

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