Sure, but behaviors that never have a bug or regression don't get a test. Software of this kind of complexity has all kinds of behavior that has never been broken, and doesn't have a specific test written for it.

Getting an extensive test suite passing is certainly orders of magnitude better than having no test suite at all, but it still doesn't tell you as much as you need to know. I would absolutely never trust an LLM Postgres rewrite (in any language) in production based on "only" Postgres's test suite passing.

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bob1029 3 hours ago | parent | next [-]

> Software of this kind of complexity has all kinds of behavior that has never been broken

This space of things is astronomically larger than the space of things expressly covered by any test suite.

"Program testing can be used to show the presence of bugs, but never to show their absence." -Edsger W. Dijkstra

	▲	mbrock 4 minutes ago \| parent \| next [-]
		The same basically holds for proofs in the absence of coherent global correctness criteria like, say, confluence and normalization for a lambda calculus, or soundness and completeness for a logic. Fable's napkin estimate of the effort required to produce a passable reference semantics for Postgres, which would involve novel discoveries in denotational semantics of concurrent transactions and so on, might be in the ballpark of 30–60 years of PhD level work. So realistically I think the only way to validate a Postgres implementation involves differential testing, fuzzing, acceptance test suites, etc. And still you'll have bugs that need to be hammered out the good old fashioned way.
	▲	w4der 3 hours ago \| parent \| prev [-]
		I've also seen situations where a customer reports a bug, the fix breaks some regression, and the updated behavior to work around the fix breaking the regressions turns into an undocumented feature.

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gblargg 3 hours ago | parent | prev | next [-]

Or even a human rewrite merely because some language is the current fad. A rewrite in a different language should be done for very good reasons, to solve problems that are bigger than the costs of all the bugs that will be introduced.

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gb2d_hn 3 hours ago | parent | prev [-]

Agreed.And a rewrite in another language creates a high probability of a change in behaviour