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Support custom field metadata in UDF #13458

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Support custom field metadata in UDF #13458

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6d5e459
Support custom schema metadata in UDF
lewiszlw Nov 18, 2024
8dadbf0
tweak doc
lewiszlw Nov 18, 2024
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3 changes: 3 additions & 0 deletions datafusion/expr/src/expr_schema.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -350,6 +350,9 @@ impl ExprSchemable for Expr {
Expr::Column(c) => Ok(schema.metadata(c)?.clone()),
Expr::Alias(Alias { expr, .. }) => expr.metadata(schema),
Expr::Cast(Cast { expr, .. }) => expr.metadata(schema),
Expr::ScalarFunction(ScalarFunction { func, args }) => {
Ok(func.metadata(args, schema))
}
_ => Ok(HashMap::new()),
}
}
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19 changes: 19 additions & 0 deletions datafusion/expr/src/udf.rs
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Expand Up @@ -28,6 +28,7 @@ use datafusion_common::{not_impl_err, ExprSchema, Result};
use datafusion_expr_common::interval_arithmetic::Interval;
use std::any::Any;
use std::cmp::Ordering;
use std::collections::HashMap;
use std::fmt::Debug;
use std::hash::{DefaultHasher, Hash, Hasher};
use std::sync::Arc;
Expand Down Expand Up @@ -216,6 +217,15 @@ impl ScalarUDF {
self.inner.is_nullable(args, schema)
}

/// Returns the field metadata for this function.
pub fn metadata(
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This feels similar in usecase to #13290 -- if we merged #13290 would that solve your usecase?

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Looks like not. This pr is for supporting custom metadata when converting scalar function expr to schema field during planing.

&self,
args: &[Expr],
schema: &dyn ExprSchema,
) -> HashMap<String, String> {
self.inner.metadata(args, schema)
}

/// Invoke the function with `args` and number of rows, returning the appropriate result.
///
/// See [`ScalarUDFImpl::invoke_batch`] for more details.
Expand Down Expand Up @@ -477,6 +487,15 @@ pub trait ScalarUDFImpl: Debug + Send + Sync {
true
}

/// Returns the field metadata for this function.
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I think a more detailed explanation would be useful for someone adding a new UDF to know why this method exists and what use case it is trying to solve. Without looking at this PR it's pretty opaque.

fn metadata(
&self,
_args: &[Expr],
_schema: &dyn ExprSchema,
) -> HashMap<String, String> {
HashMap::new()
}

/// Invoke the function on `args`, returning the appropriate result
///
/// The function will be invoked passed with the slice of [`ColumnarValue`]
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