adriangb commented on code in PR #15801:
URL: https://github.com/apache/datafusion/pull/15801#discussion_r2070247089
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datafusion/physical-plan/src/execution_plan.rs:
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@@ -471,39 +471,53 @@ pub trait ExecutionPlan: Debug + DisplayAs + Send + Sync {
Ok(None)
}
- /// Attempts to recursively push given filters from the top of the tree
into leafs.
- ///
- /// This is used for various optimizations, such as:
- ///
- /// * Pushing down filters into scans in general to minimize the amount of
data that needs to be materialzied.
- /// * Pushing down dynamic filters from operators like TopK and Joins into
scans.
- ///
- /// Generally the further down (closer to leaf nodes) that filters can be
pushed, the better.
- ///
- /// Consider the case of a query such as `SELECT * FROM t WHERE a = 1 AND
b = 2`.
- /// With no filter pushdown the scan needs to read and materialize all the
data from `t` and then filter based on `a` and `b`.
- /// With filter pushdown into the scan it can first read only `a`, then
`b` and keep track of
- /// which rows match the filter.
- /// Then only for rows that match the filter does it have to materialize
the rest of the columns.
- ///
- /// # Default Implementation
- ///
- /// The default implementation assumes:
- /// * Parent filters can't be passed onto children.
- /// * This node has no filters to contribute.
- ///
- /// # Implementation Notes
- ///
- /// Most of the actual logic is implemented as a Physical Optimizer rule.
- /// See [`PushdownFilter`] for more details.
- ///
- /// [`PushdownFilter`]:
https://docs.rs/datafusion/latest/datafusion/physical_optimizer/filter_pushdown/struct.PushdownFilter.html
- fn try_pushdown_filters(
Review Comment:
Agreed I'd prefer a single method as well - but as per
https://github.com/apache/datafusion/pull/15770#discussion_r2052311257 we
probably would have had to add a new method to the existing implementation
anyway. I don't see a way to have a single method without making it absurdly
convoluted just to avoid 2 methods.
I'll also point out that there are multiple dimensions of filter pushdown
for each node:
- does the node allow parent filters to be pushed down to it's children?
- does the node want to add any filters to be passed to it's children?
- does the node need to handle the result of pushdown?
Having two methods makes it easy to have canned implementations for each one
of these independently and combine/compose them.
E.g.:
- TopK or HashJoin will override `gather_filters_for_pushdown` but not
`handle_child_pushdown_result`
- FilterExec will override both
- DataSourceExec will override only `handle_child_pushdown_result`
- ProjectionExec will override only `gather_filters_for_pushdown`
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