cisaacson commented on code in PR #9988:
URL: https://github.com/apache/arrow-datafusion/pull/9988#discussion_r1556329614
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datafusion/core/src/datasource/provider.rs:
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@@ -161,20 +161,54 @@ 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:
+ ///
+ /// ```
+ /// fn supports_filters_pushdown(
+ /// &self,
+ /// filters: &[&Expr],) -> Result<Vec<TableProviderFilterPushDown>> {
+ ///
+ /// let result_vec: Vec<TableProviderFilterPushDown> =
Vec::with_capacity(&filters.len());
+ /// for i in 0..filters.len() {
+ /// // Evaluate a filter
+ /// let filter = filters[i];
+ ///
+ /// // Evaluate a filter to support here
+ /// if filter ... {
+ /// result_vec.push(TableProviderFilterPushDown::Exact);
+ /// } else {
+ /// result_vec.push(TableProviderFilterPushDown::Unsupported);
+ /// }
+ /// }
Review Comment:
Let me know if the latest commit looks OK to you. I didn't see the
functional style example until just now, but my example does compile. I bet you
may have a better way to do the whole thing, I'm interested in that feedback
too. I can look at this again this evening. And I could easily convert it to
functional style too. If I put another fn in to evaluate each filter (which is
how I would do it) then that would have to be in the example too? That will
make it a lot longer so I put just one Expr eval to show the concept.
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