alamb commented on code in PR #9988:
URL: https://github.com/apache/arrow-datafusion/pull/9988#discussion_r1556334496
##########
datafusion/core/src/datasource/provider.rs:
##########
@@ -161,20 +161,71 @@ pub trait TableProvider: Sync + Send {
/// Specify if DataFusion should provide filter expressions to the
/// TableProvider to apply *during* the scan.
///
- /// The return value must have one element for each filter expression
passed
- /// in. The value of each element indicates if the TableProvider can apply
- /// that particular filter during the scan.
- ///
/// Some TableProviders can evaluate filters more efficiently than the
/// `Filter` operator in DataFusion, for example by using an index.
- ///
- /// By default, returns [`Unsupported`] for all filters, meaning no filters
+ ///
+ /// The return value must have one element for each filter expression
passed
+ /// in. The value of each element indicates if the TableProvider can apply
+ /// that particular filter during the scan. The position in the return
value
+ /// Vec corresponds to the expression in the `filters` input.
+ ///
+ /// Here is an example of how this can be done:
+ ///
+ /// ```rust
+ /// use datafusion::error::{Result, DataFusionError};
+ /// use datafusion_expr::{Expr, TableProviderFilterPushDown};
+ ///
+ /// fn supports_filters_pushdown(
+ /// &self,
+ /// filters: &[&Expr],) -> Result<Vec<TableProviderFilterPushDown>> {
+ ///
+ /// let mut result_vec: Vec<TableProviderFilterPushDown> =
Vec::with_capacity(&filters.len());
+ ///
+ /// for i in 0..filters.len() {
+ /// let expr = *filters[i];
+ ///
+ /// match expr {
+ /// Expr::Between(between_expr) => {
+ /// match between_expr.expr.try_into_col().ok() {
+ /// Some(column) => {
+ /// let column_name = column.name;
+ /// if column_name = "c1".to_string() {
+ ///
result_vec.push(TableProviderFilterPushDown::Exact);
+ /// } else {
+ ///
result_vec.push(TableProviderFilterPushDown::Unsupported);
+ /// }
+ /// }
+ /// None => {
+ /// return
Err(DataFusionError::Execution(format!("Could not get column. between_expr:
{:?}", between_expr)));
+ /// }
Review Comment:
The example might be more general if it didn't error but instead returned
`Unsupported` for other exprs
Also, if you want to get your functional Rust programming on, you can use
something like this (untested):
```rust
between_expr.expr.try_into_col()
.map(|column| {
if &column.name = "c1" {
TableProviderFilterPushDown::Exact
} else {
TableProviderFilterPushDown::Unsupported
}
})
// if not a column reference, return unsupported
.unwrap_or(TableProviderFilterPushDown::Unsupported)
```
##########
datafusion/core/src/datasource/provider.rs:
##########
@@ -161,20 +161,71 @@ pub trait TableProvider: Sync + Send {
/// Specify if DataFusion should provide filter expressions to the
/// TableProvider to apply *during* the scan.
///
- /// The return value must have one element for each filter expression
passed
- /// in. The value of each element indicates if the TableProvider can apply
- /// that particular filter during the scan.
- ///
/// Some TableProviders can evaluate filters more efficiently than the
/// `Filter` operator in DataFusion, for example by using an index.
- ///
- /// By default, returns [`Unsupported`] for all filters, meaning no filters
+ ///
+ /// The return value must have one element for each filter expression
passed
+ /// in. The value of each element indicates if the TableProvider can apply
+ /// that particular filter during the scan. The position in the return
value
+ /// Vec corresponds to the expression in the `filters` input.
+ ///
+ /// Here is an example of how this can be done:
+ ///
+ /// ```rust
+ /// use datafusion::error::{Result, DataFusionError};
+ /// use datafusion_expr::{Expr, TableProviderFilterPushDown};
+ ///
+ /// fn supports_filters_pushdown(
+ /// &self,
+ /// filters: &[&Expr],) -> Result<Vec<TableProviderFilterPushDown>> {
+ ///
+ /// let mut result_vec: Vec<TableProviderFilterPushDown> =
Vec::with_capacity(&filters.len());
+ ///
+ /// for i in 0..filters.len() {
+ /// let expr = *filters[i];
+ ///
+ /// match expr {
Review Comment:
For examples, I think it helps to add a comment explaining what it is doing
for new readers
for example, here you might add:
```suggestion
/// // Return `Exact` if we have a way to evaluate the
expression exactly. In this
/// // we have a way to evaluate expressions on column c1, but
not other columns,
/// match expr {
```
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