alamb commented on code in PR #17264:
URL: https://github.com/apache/datafusion/pull/17264#discussion_r2292196513
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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.
Immutable,
- /// A stable function may return different values given the same input
across different
- /// queries but must return the same value for a given input within a
query. An example of
- /// this is the `Now` function. DataFusion will attempt to inline `Stable`
functions
- /// during planning, when possible.
- /// For query `select col1, now() from t1`, it might take a while to
execute but
- /// `now()` column will be the same for each output row, which is evaluated
- /// during planning.
+ /// May return different values given the same input across different
+ /// queries but must return the same value for a given input within a
query.
+ ///
+ /// An example of a stable function is the `now()` function. For example,
+ /// the query `select col1, now() from t1`, will return different results
+ /// each time it is run, but within the same query, the output of the
+ /// `now()` function has the same value for each output row.
+ ///
+ /// DataFusion will inline `Stable` functions during planning, when
Review Comment:
For example, `Stable` functions are inlined when planning a query for
execution, but not in View definitions or prepared statements. But `Immutable`
functions are always inlined when possible
I have tried to clarify as well
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