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Posted to github@arrow.apache.org by "mustafasrepo (via GitHub)" <gi...@apache.org> on 2023/06/10 10:31:39 UTC
[GitHub] [arrow-datafusion] mustafasrepo opened a new pull request, #6625: Add hash support for PhysicalExpr and PhysicalSortExpr
mustafasrepo opened a new pull request, #6625:
URL: https://github.com/apache/arrow-datafusion/pull/6625
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In the [discussion](https://github.com/apache/arrow-datafusion/pull/6501#pullrequestreview-1458404791) @alamb suggested to implement `dyn_hash` method so that we can add support for hashing to traits. In this PR by using his suggestions we added support for hashing to `PhysicalExpr` and `PhysicalSortExpr `. This enables us to use `HashSet` inside `EquivalenceClass`. Hence logic is simplified a bit in `EquivalenceClass` also.
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[GitHub] [arrow-datafusion] alamb merged pull request #6625: Add hash support for PhysicalExpr and PhysicalSortExpr
Posted by "alamb (via GitHub)" <gi...@apache.org>.
alamb merged PR #6625:
URL: https://github.com/apache/arrow-datafusion/pull/6625
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[GitHub] [arrow-datafusion] alamb commented on a diff in pull request #6625: Add hash support for PhysicalExpr and PhysicalSortExpr
Posted by "alamb (via GitHub)" <gi...@apache.org>.
alamb commented on code in PR #6625:
URL: https://github.com/apache/arrow-datafusion/pull/6625#discussion_r1225823671
##########
datafusion/physical-expr/src/expressions/cast.rs:
##########
@@ -132,6 +133,12 @@ impl PhysicalExpr for CastExpr {
interval.cast_to(&cast_type, &self.cast_options)?,
)])
}
+
+ fn dyn_hash(&self, _state: &mut dyn Hasher) {
+ // `self.cast_options` doesn't support hashing
+ // Hence we cannot calculate `dyn_hash` for this type.
+ unimplemented!();
Review Comment:
I filed https://github.com/apache/arrow-rs/pull/4395 to add this upstream
Rather than `unimplmented` which will panic if this code is ever run (for eample if someone tries to put a CastExpr into a `HashSet`), I think it is ok to hash the other fields while we wait for upstream.
The requirements on hash https://doc.rust-lang.org/std/hash/trait.Hash.html#hash-and-eq are
> In other words, if two keys are equal, their hashes must also be equal.
Which I believe this implementation would satisfy:
```rust
impl Hash for CastExpr {
fn hash<H: Hasher>(&self, state: &mut H) {
self.expr.hash(state);
self.cast_type.hash(state);
// Add self.cast_options when hash is available
// https://github.com/apache/arrow-rs/pull/4395
}
}
```
##########
datafusion/physical-expr/src/equivalence.rs:
##########
@@ -39,7 +40,7 @@ pub struct EquivalenceProperties<T = Column> {
schema: SchemaRef,
}
-impl<T: PartialEq + Clone> EquivalenceProperties<T> {
+impl<T: Eq + Clone + Hash> EquivalenceProperties<T> {
Review Comment:
❤️
##########
datafusion/physical-expr/src/scalar_function.rs:
##########
@@ -162,6 +163,12 @@ impl PhysicalExpr for ScalarFunctionExpr {
self.return_type(),
)))
}
+
+ fn dyn_hash(&self, _state: &mut dyn Hasher) {
+ // hashing for `self.fun` is not supported
Review Comment:
likewise here can we create the `Hash` impl instead?
##########
datafusion/physical-expr/src/physical_expr.rs:
##########
@@ -104,6 +105,14 @@ pub trait PhysicalExpr: Send + Sync + Display + Debug + PartialEq<dyn Any> {
"Not implemented for {self}"
)))
}
+
+ fn dyn_hash(&self, _state: &mut dyn Hasher);
Review Comment:
I think we should add some comments here to help anyone who tries to implement `PhysicalExpr`
Perhaps we can follow the model of the documentation on
https://docs.rs/datafusion-expr/26.0.0/datafusion_expr/logical_plan/trait.UserDefinedLogicalNode.html#tymethod.dyn_hash
##########
datafusion/physical-expr/src/expressions/in_list.rs:
##########
@@ -330,6 +331,12 @@ impl PhysicalExpr for InListExpr {
self.static_filter.clone(),
)))
}
+
+ fn dyn_hash(&self, _state: &mut dyn Hasher) {
Review Comment:
The same comment from above here applies -- I think a less precise hash function (with more collisions) is better than a runtime panic. Thus I think we should implement `Hash` for `static_filter`
##########
datafusion/physical-expr/src/expressions/cast.rs:
##########
@@ -132,6 +133,12 @@ impl PhysicalExpr for CastExpr {
interval.cast_to(&cast_type, &self.cast_options)?,
)])
}
+
+ fn dyn_hash(&self, _state: &mut dyn Hasher) {
+ // `self.cast_options` doesn't support hashing
+ // Hence we cannot calculate `dyn_hash` for this type.
+ unimplemented!();
Review Comment:
I filed https://github.com/apache/arrow-rs/pull/4395 to add this upstream
Rather than `unimplmented` which will panic if this code is ever run (for eample if someone tries to put a CastExpr into a `HashSet`), I think it is ok to hash the other fields while we wait for upstream.
The requirements on hash https://doc.rust-lang.org/std/hash/trait.Hash.html#hash-and-eq are
> In other words, if two keys are equal, their hashes must also be equal.
Which I believe this implementation would satisfy (though it will have some more hash collisions than when it would include `cast_options`):
```rust
impl Hash for CastExpr {
fn hash<H: Hasher>(&self, state: &mut H) {
self.expr.hash(state);
self.cast_type.hash(state);
// Add self.cast_options when hash is available
// https://github.com/apache/arrow-rs/pull/4395
}
}
```
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