What is meant by a non-volatile expression in Postgresql?

Question

Reading this post, the author references a "non-volatile" expression, after researching a bit, I'm still not sure what it is. What is meant by a non-volatile expression in Postgresql?

A trip to the docco might be helpful. – Vérace Dec 16 '21 at 08:16 — Vérace, Dec 16 '21 at 08:16

Erwin Brandstetter · Accepted Answer · 2022-01-04T20:53:21.430

"Immutable" expressions resolve to the same value always. Like 2 + 3.

"Stable" expressions resolve to the same value within a single table scan. Like: now().

"Volatile" expressions can resolve to a different value for each evaluation. Like: clock_timestamp().

"Non-volatile" would be in one of the first two categories.

What is meant by a non-volatile expression in Postgresql?

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