Josh Berkus writes:
I'll also point out that some of our settings only really "work" in
combinations of two or more settings. For example, one doesn't want to
set archive_mode = on unless one is setting archive_command as well.
And generally if one sets sequential_page_cost, one is changing the
other cost parameters as well. And logging parameters are generally
managed as a set.
So the case of two sessions both modifying ALTER SYSTEM SET, and one
succeeding for some-but-all-GUCS, and the other succeeding for
some-but-not-all-GUCs, would not be user-friendly or pretty, even if
each setting change succeeded or failed atomically.
That is a killer point. So really the value of the global lock is to
ensure serializability when transactions are updating multiple GUCs.

    regards, tom lane

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