2010YOUY01 commented on code in PR #17264:
URL: https://github.com/apache/datafusion/pull/17264#discussion_r2290507219
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datafusion/expr-common/src/signature.rs:
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@@ -44,42 +43,79 @@ pub const TIMEZONE_WILDCARD: &str = "+TZ";
/// valid length. It exists to avoid the need to enumerate all possible fixed
size list lengths.
pub const FIXED_SIZE_LIST_WILDCARD: i32 = i32::MIN;
-/// A function's volatility, which defines the functions eligibility for
certain optimizations
+/// When a function's output changes when the input does not
+///
+/// The volatility of a function determine eligibility for certain
+/// optimizations. You should always defined your function to have the
strictest
+/// volatility that applies to it to maximize performance and avoid unexpected
+/// results.
+///
#[derive(Debug, PartialEq, Eq, PartialOrd, Ord, Clone, Copy, Hash)]
pub enum Volatility {
- /// An immutable function will always return the same output when given
the same
- /// input. DataFusion will attempt to inline immutable functions during
planning.
+ /// Always returns the same output when given the same input.
+ ///
+ /// DataFusion will inline immutable functions during planning.
Review Comment:
I think it's like replacing `upper("hello")` with `"HELLO"` during planning.
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