alamb commented on code in PR #10560:
URL: https://github.com/apache/datafusion/pull/10560#discussion_r1630300241
##########
datafusion/functions-aggregate/src/first_last.rs:
##########
@@ -31,20 +31,24 @@ use datafusion_common::{
use datafusion_expr::function::{AccumulatorArgs, StateFieldsArgs};
use datafusion_expr::utils::{format_state_name, AggregateOrderSensitivity};
use datafusion_expr::{
- Accumulator, AggregateUDFImpl, ArrayFunctionSignature, Signature,
TypeSignature,
- Volatility,
+ Accumulator, AggregateUDFExprBuilder, AggregateUDFImpl,
ArrayFunctionSignature, Expr,
+ Signature, TypeSignature, Volatility,
};
use datafusion_physical_expr_common::aggregate::utils::get_sort_options;
use datafusion_physical_expr_common::sort_expr::{
limited_convert_logical_sort_exprs_to_physical, LexOrdering,
PhysicalSortExpr,
};
-make_udaf_expr_and_func!(
- FirstValue,
- first_value,
- "Returns the first value in a group of values.",
- first_value_udaf
-);
+create_func!(FirstValue, first_value_udaf);
+
+/// Returns the first value in a group of values.
+pub fn first_value(expression: Expr, order_by: Option<Vec<Expr>>) -> Expr {
+ if let Some(order_by) = order_by {
Review Comment:
that is pretty cool indeed
##########
datafusion-examples/examples/udaf_expr.rs:
##########
@@ -0,0 +1,44 @@
+// Licensed to the Apache Software Foundation (ASF) under one
+// or more contributor license agreements. See the NOTICE file
+// distributed with this work for additional information
+// regarding copyright ownership. The ASF licenses this file
+// to you under the Apache License, Version 2.0 (the
+// "License"); you may not use this file except in compliance
+// with the License. You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing,
+// software distributed under the License is distributed on an
+// "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+// KIND, either express or implied. See the License for the
+// specific language governing permissions and limitations
+// under the License.
+
+use datafusion::{
+ execution::{
+ config::SessionConfig,
+ context::{SessionContext, SessionState},
+ },
+ functions_aggregate::{expr_fn::first_value, register_all},
+};
+
+use datafusion_common::Result;
+use datafusion_expr::{col, AggregateUDFExprBuilder};
+
+#[tokio::main]
+async fn main() -> Result<()> {
+ let ctx = SessionContext::new();
+ let config = SessionConfig::new();
+ let mut state = SessionState::new_with_config_rt(config,
ctx.runtime_env());
+ let _ = register_all(&mut state);
+
+ let first_value_udaf =
state.aggregate_functions().get("first_value").unwrap();
+ let first_value_builder = first_value_udaf
+ .call(vec![col("a")])
+ .order_by(vec![col("b")]);
Review Comment:
I think it would help to add comments to this example explaining what it was
trying to show
Something like
```rust
// DataFusion also supports more advanced aggregate expressions, like
// `FIRST_VALUE(a ORDER BY b)`. To create such an expression using the
expr_api
// you can use `AggregateUDFExprBuilder` trait as follows:
```
Also, I think it would actually be easier to find as a function in
https://github.com/apache/datafusion/blob/main/datafusion-examples/examples/expr_api.rs
rather than its own separate example
##########
datafusion/expr/src/udaf.rs:
##########
@@ -606,3 +607,49 @@ impl AggregateUDFImpl for AggregateUDFLegacyWrapper {
(self.accumulator)(acc_args)
}
}
+
+pub trait AggregateUDFExprBuilder {
Review Comment:
Also, since it works with `Expr` (not just `AggregateUDF`) I wonder if we
should call it something more general like `AggregateExprBuilder` 🤔
Also, I think we should document this trait with some examples, and
specifically documenting the behavior of what happens when the Expr is *not*
an `Expr::AggregateFunction` (the code silently ignores the expr)
Something like this maybe
```rust
/// Trait to assist building aggregate expressions.
///
/// # Error handing
/// (what happens if the Expr is not an aggregate)
///
/// # Example
/// ```
/// show it in use
/// ```
```
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