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// Copyright Materialize, Inc. and contributors. All rights reserved.
//
// Use of this software is governed by the Business Source License
// included in the LICENSE file.
//
// As of the Change Date specified in that file, in accordance with
// the Business Source License, use of this software will be governed
// by the Apache License, Version 2.0.
//! Install replace certain `Get` operators with their `Let` value.
//!
//! Some `Let` bindings are not useful, for example when they bind
//! a `Get` as their value, or when there is a single corresponding
//! `Get` statement in their body. These cases can be inlined without
//! harming planning.
use crate::TransformArgs;
use mz_expr::visit::{Visit, VisitChildren};
use mz_expr::{Id, LocalId, MirRelationExpr, RECURSION_LIMIT};
use mz_ore::stack::{CheckedRecursion, RecursionGuard};
/// Install replace certain `Get` operators with their `Let` value.
#[derive(Debug)]
pub struct InlineLet {
/// If `true`, inline MFPs around a Get.
///
/// We want this value to be true for the InlineLet call that comes right
/// before [crate::join_implementation::JoinImplementation] runs because
/// [crate::join_implementation::JoinImplementation] cannot lift MFPs
/// through a Let.
///
/// Generally, though, we prefer to be more conservative in our inlining in
/// order to be able to better detect CSEs.
pub inline_mfp: bool,
/// A [`RecursionGuard`] to be used by the [`CheckedRecursion`] implementation.
recursion_guard: RecursionGuard,
}
impl InlineLet {
/// Construct a new [`InlineLet`] instance with the given `inline_mfp`
/// where `recursion_guard` is initialized with [`RECURSION_LIMIT`] as limit.
pub fn new(inline_mfp: bool) -> InlineLet {
InlineLet {
inline_mfp,
recursion_guard: RecursionGuard::with_limit(RECURSION_LIMIT),
}
}
}
impl CheckedRecursion for InlineLet {
fn recursion_guard(&self) -> &RecursionGuard {
&self.recursion_guard
}
}
impl crate::Transform for InlineLet {
#[tracing::instrument(
target = "optimizer"
level = "trace",
skip_all,
fields(path.segment = "inline_let")
)]
fn transform(
&self,
relation: &mut MirRelationExpr,
_: TransformArgs,
) -> Result<(), crate::TransformError> {
let result = self.transform_without_trace(relation);
mz_repr::explain_new::trace_plan(&*relation);
result
}
}
impl InlineLet {
/// Performs the `InlineLet` transformation without tracing the result.
pub fn transform_without_trace(
&self,
relation: &mut MirRelationExpr,
) -> Result<(), crate::TransformError> {
let mut lets = vec![];
self.action(relation, &mut lets)?;
for (id, value) in lets.into_iter().rev() {
*relation = MirRelationExpr::Let {
id,
value: Box::new(value),
body: Box::new(relation.take_dangerous()),
};
}
Ok(())
}
/// Install replace certain `Get` operators with their `Let` value.
///
/// IMPORTANT: This transform is used for cleaning up after `RelationCSE`, which
/// adds `Let` operators pretty aggressively, leading to very deep dataflows. Nothing
/// in this transform should lead to expensive recursive traversal of the subgraph,
/// such as the one in `MirRelationExpr::typ`, since that may result in a stack
/// overflow.
pub fn action(
&self,
relation: &mut MirRelationExpr,
lets: &mut Vec<(LocalId, MirRelationExpr)>,
) -> Result<(), crate::TransformError> {
self.checked_recur(|_| {
if let MirRelationExpr::Let { id, value, body } = relation {
self.action(value, lets)?;
let mut num_gets = 0;
body.try_visit_mut_pre::<_, crate::TransformError>(
&mut |relation| match relation {
MirRelationExpr::Get { id: get_id, .. } if Id::Local(*id) == *get_id => {
num_gets += 1;
Ok(())
}
_ => Ok(()),
},
)?;
let stripped_value = if self.inline_mfp {
mz_expr::MapFilterProject::extract_non_errors_from_expr(&**value).1
} else {
&**value
};
let inlinable = match stripped_value {
MirRelationExpr::Get { .. } | MirRelationExpr::Constant { .. } => true,
_ => num_gets <= 1,
};
if inlinable {
// if only used once, just inline it
body.try_visit_mut_pre::<_, crate::TransformError>(&mut |relation| {
match relation {
MirRelationExpr::Get { id: get_id, .. }
if Id::Local(*id) == *get_id =>
{
*relation = (**value).clone();
Ok(())
}
_ => Ok(()),
}
})?;
} else {
// otherwise lift it to the top so it's out of the way
lets.push((*id, value.take_dangerous()));
}
*relation = body.take_dangerous();
// might be another Let in the body so have to recur here
self.action(relation, lets)
} else {
relation.try_visit_mut_children(|child| self.action(child, lets))
}
})
}
}