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

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