Improve documentation and add examples for ArrowPredicateFn (#7480)

Author: alamb

parquet/src/arrow/arrow_reader/filter.rs

@@ -21,11 +21,18 @@ use arrow_schema::ArrowError;
 
 /// A predicate operating on [`RecordBatch`]
 ///
-/// See [`RowFilter`] for more information on the use of this trait.
+/// See also:
+/// * [`RowFilter`] for more information  on applying filters during the
+///   Parquet decoding process.
+/// * [`ArrowPredicateFn`] for a concrete implementation based on a function
 pub trait ArrowPredicate: Send + 'static {
     /// Returns the [`ProjectionMask`] that describes the columns required
-    /// to evaluate this predicate. All projected columns will be provided in the `batch`
-    /// passed to [`evaluate`](Self::evaluate)
+    /// to evaluate this predicate.
+    ///
+    /// All projected columns will be provided in the `batch` passed to
+    /// [`evaluate`](Self::evaluate). The projection mask should be as small as
+    /// possible because any columns needed for the overall projection mask are
+    /// decoded again after a predicate is applied.
     fn projection(&self) -> &ProjectionMask;
 
     /// Evaluate this predicate for the given [`RecordBatch`] containing the columns
@@ -38,7 +45,63 @@ pub trait ArrowPredicate: Send + 'static {
     fn evaluate(&mut self, batch: RecordBatch) -> Result<BooleanArray, ArrowError>;
 }
 
-/// An [`ArrowPredicate`] created from an [`FnMut`]
+/// An [`ArrowPredicate`] created from an [`FnMut`] and a [`ProjectionMask`]
+///
+/// See [`RowFilter`] for more information on applying filters during the
+/// Parquet decoding process.
+///
+/// The function is passed `RecordBatch`es with only the columns specified in
+/// the [`ProjectionMask`].
+///
+/// The function must return a [`BooleanArray`] that has the same length as the
+/// input `batch` where each row indicates whether the row should be returned:
+/// * `true`: the row should be returned
+/// * `false` or `null`: the row should not be returned
+///
+/// # Example:
+///
+/// Given an input schema: `"a:int64", "b:int64"`, you can create a predicate that
+/// evaluates `b > 0` like this:
+///
+/// ```
+/// # use std::sync::Arc;
+/// # use arrow::compute::kernels::cmp::gt;
+/// # use arrow_array::{BooleanArray, Int64Array, RecordBatch};
+/// # use arrow_array::cast::AsArray;
+/// # use arrow_array::types::Int64Type;
+/// # use parquet::arrow::arrow_reader::ArrowPredicateFn;
+/// # use parquet::arrow::ProjectionMask;
+/// # use parquet::schema::types::{SchemaDescriptor, Type};
+/// # use parquet::basic; // note there are two `Type`s that are different
+/// # // Schema for a table with one columns: "a" (int64) and "b" (int64)
+/// # let descriptor = SchemaDescriptor::new(
+/// #  Arc::new(
+/// #    Type::group_type_builder("my_schema")
+/// #      .with_fields(vec![
+/// #        Arc::new(
+/// #         Type::primitive_type_builder("a", basic::Type::INT64)
+/// #          .build().unwrap()
+/// #        ),
+/// #        Arc::new(
+/// #         Type::primitive_type_builder("b", basic::Type::INT64)
+/// #          .build().unwrap()
+/// #        ),
+/// #     ])
+/// #     .build().unwrap()
+/// #  )
+/// # );
+/// // Create a mask for selecting only the second column "b" (index 1)
+/// let projection_mask = ProjectionMask::leaves(&descriptor, [1]);
+/// // Closure that evaluates "b > 0"
+/// let predicate = |batch: RecordBatch| {
+///    let scalar_0 = Int64Array::new_scalar(0);
+///    let column = batch.column(0).as_primitive::<Int64Type>();
+///    // call the gt kernel to compute `>` which returns a BooleanArray
+///    gt(column, &scalar_0)
+///  };
+/// // Create ArrowPredicateFn that can be passed to RowFilter
+/// let arrow_predicate = ArrowPredicateFn::new(projection_mask, predicate);
+/// ```
 pub struct ArrowPredicateFn<F> {
     f: F,
     projection: ProjectionMask,
@@ -48,10 +111,8 @@ impl<F> ArrowPredicateFn<F>
 where
     F: FnMut(RecordBatch) -> Result<BooleanArray, ArrowError> + Send + 'static,
 {
-    /// Create a new [`ArrowPredicateFn`]. `f` will be passed batches
-    /// that contains the columns specified in `projection`
-    /// and returns a [`BooleanArray`] that describes which rows should
-    /// be passed along
+    /// Create a new [`ArrowPredicateFn`] that invokes `f` on the columns
+    /// specified in `projection`.
     pub fn new(projection: ProjectionMask, f: F) -> Self {
         Self { f, projection }
     }