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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.
//! Transformations for relation expressions.
//!
//! This crate contains traits, types, and methods suitable for transforming
//! `MirRelationExpr` types in ways that preserve semantics and improve performance.
//! The core trait is `Transform`, and many implementors of this trait can be
//! boxed and iterated over. Some common transformation patterns are wrapped
//! as `Transform` implementors themselves.
//!
//! The crate also contains the beginnings of whole-dataflow optimization,
//! which uses the same analyses but spanning multiple dataflow elements.
#![warn(missing_docs)]
#![warn(missing_debug_implementations)]
use std::collections::HashMap;
use std::error::Error;
use std::fmt;
use expr::GlobalId;
use expr::MirRelationExpr;
use expr::MirScalarExpr;
use ore::id_gen::IdGen;
pub mod canonicalize_mfp;
pub mod column_knowledge;
pub mod cse;
pub mod demand;
pub mod fusion;
pub mod inline_let;
pub mod join_implementation;
pub mod map_lifting;
pub mod monotonic;
pub mod nonnull_requirements;
pub mod nonnullable;
pub mod predicate_pushdown;
pub mod projection_extraction;
pub mod projection_lifting;
pub mod projection_pushdown;
pub mod reduce_elision;
pub mod reduction;
pub mod reduction_pushdown;
pub mod redundant_join;
pub mod topk_elision;
pub mod union_cancel;
pub mod update_let;
pub mod dataflow;
pub use dataflow::optimize_dataflow;
use ore::stack::RecursionLimitError;
/// Arguments that get threaded through all transforms.
#[derive(Debug)]
pub struct TransformArgs<'a> {
/// A shared instance of IdGen to allow constructing new Let expressions.
pub id_gen: &'a mut IdGen,
/// The indexes accessible.
pub indexes: &'a HashMap<GlobalId, Vec<(GlobalId, Vec<MirScalarExpr>)>>,
}
/// Types capable of transforming relation expressions.
pub trait Transform: std::fmt::Debug {
/// Transform a relation into a functionally equivalent relation.
fn transform(
&self,
relation: &mut MirRelationExpr,
args: TransformArgs,
) -> Result<(), TransformError>;
/// A string describing the transform.
///
/// This is useful mainly when iterating through many `Box<Tranform>`
/// and one wants to judge progress before some defect occurs.
fn debug(&self) -> String {
format!("{:?}", self)
}
}
/// Errors that can occur during a transformation.
#[derive(Debug, Clone)]
pub enum TransformError {
/// An unstructured error.
Internal(String),
}
impl fmt::Display for TransformError {
fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
match self {
TransformError::Internal(msg) => write!(f, "internal transform error: {}", msg),
}
}
}
impl Error for TransformError {}
impl From<RecursionLimitError> for TransformError {
fn from(error: RecursionLimitError) -> Self {
TransformError::Internal(error.to_string())
}
}
/// A sequence of transformations iterated some number of times.
#[derive(Debug)]
pub struct Fixpoint {
transforms: Vec<Box<dyn crate::Transform>>,
limit: usize,
}
impl Transform for Fixpoint {
fn transform(
&self,
relation: &mut MirRelationExpr,
args: TransformArgs,
) -> Result<(), TransformError> {
// The number of iterations for a relation to settle depends on the
// number of nodes in the relation. Instead of picking an arbitrary
// hard limit on the number of iterations, we use a soft limit and
// check whether the relation has become simpler after reaching it.
// If so, we perform another pass of transforms. Otherwise, there is
// a bug somewhere that prevents the relation from settling on a
// stable shape.
loop {
let mut original_count = 0;
relation.try_visit_post::<_, TransformError>(&mut |_| Ok(original_count += 1))?;
for _ in 0..self.limit {
let original = relation.clone();
for transform in self.transforms.iter() {
transform.transform(
relation,
TransformArgs {
id_gen: args.id_gen,
indexes: args.indexes,
},
)?;
}
if *relation == original {
return Ok(());
}
}
let mut final_count = 0;
relation.try_visit_post::<_, TransformError>(&mut |_| Ok(final_count += 1))?;
if final_count >= original_count {
break;
}
}
for transform in self.transforms.iter() {
transform.transform(
relation,
TransformArgs {
id_gen: args.id_gen,
indexes: args.indexes,
},
)?;
}
Err(TransformError::Internal(format!(
"fixpoint looped too many times {:#?}; transformed relation: {}",
self,
relation.pretty()
)))
}
}
/// A sequence of transformations that simplify the `MirRelationExpr`
#[derive(Debug)]
pub struct FuseAndCollapse {
transforms: Vec<Box<dyn crate::Transform>>,
}
impl Default for FuseAndCollapse {
fn default() -> Self {
Self {
// TODO: The relative orders of the transforms have not been
// determined except where there are comments.
// TODO (#6542): All the transforms here except for
// `ProjectionLifting`, `InlineLet`, `UpdateLet`, and
// `RedundantJoin` can be implemented as free functions. Note that
// (#716) proposes the removal of `InlineLet` and `UpdateLet` as a
// transforms.
transforms: vec![
Box::new(crate::projection_extraction::ProjectionExtraction),
Box::new(crate::projection_lifting::ProjectionLifting::default()),
Box::new(crate::fusion::map::Map),
Box::new(crate::fusion::negate::Negate),
Box::new(crate::fusion::filter::Filter),
Box::new(crate::fusion::project::Project),
Box::new(crate::fusion::join::Join),
Box::new(crate::fusion::top_k::TopK),
Box::new(crate::inline_let::InlineLet::new(false)),
Box::new(crate::fusion::reduce::Reduce),
Box::new(crate::fusion::union::Union),
// This goes after union fusion so we can cancel out
// more branches at a time.
Box::new(crate::union_cancel::UnionBranchCancellation),
// This should run before redundant join to ensure that key info
// is correct.
Box::new(crate::update_let::UpdateLet::default()),
// Removes redundant inputs from joins.
// Note that this eliminates one redundant input per join,
// so it is necessary to run this section in a loop.
// TODO: (#6748) Predicate pushdown unlocks the ability to
// remove some redundant joins but also prevents other
// redundant joins from being removed. When predicate pushdown
// no longer works against redundant join, check if it is still
// necessary to run RedundantJoin here.
Box::new(crate::redundant_join::RedundantJoin::default()),
// As a final logical action, convert any constant expression to a constant.
// Some optimizations fight against this, and we want to be sure to end as a
// `MirRelationExpr::Constant` if that is the case, so that subsequent use can
// clearly see this.
Box::new(crate::reduction::FoldConstants { limit: Some(10000) }),
],
}
}
}
impl Transform for FuseAndCollapse {
fn transform(
&self,
relation: &mut MirRelationExpr,
args: TransformArgs,
) -> Result<(), TransformError> {
for transform in self.transforms.iter() {
transform.transform(
relation,
TransformArgs {
id_gen: args.id_gen,
indexes: args.indexes,
},
)?;
}
Ok(())
}
}
/// A naive optimizer for relation expressions.
///
/// The optimizer currently applies only peep-hole optimizations, from a limited
/// set that were sufficient to get some of TPC-H up and working. It is worth a
/// review at some point to improve the quality, coverage, and architecture of
/// the optimizations.
#[derive(Debug)]
pub struct Optimizer {
/// The list of transforms to apply to an input relation.
pub transforms: Vec<Box<dyn crate::Transform>>,
}
impl Optimizer {
/// Builds a logical optimizer that only performs logical transformations.
pub fn logical_optimizer() -> Self {
let transforms: Vec<Box<dyn crate::Transform>> = vec![
// 1. Structure-agnostic cleanup
Box::new(crate::topk_elision::TopKElision),
Box::new(crate::nonnull_requirements::NonNullRequirements::default()),
// 2. Collapse constants, joins, unions, and lets as much as possible.
// TODO: lift filters/maps to maximize ability to collapse
// things down?
Box::new(crate::Fixpoint {
limit: 100,
transforms: vec![Box::new(crate::FuseAndCollapse::default())],
}),
// 3. Move predicate information up and down the tree.
// This also fixes the shape of joins in the plan.
Box::new(crate::Fixpoint {
limit: 100,
transforms: vec![
// Predicate pushdown sets the equivalence classes of joins.
Box::new(crate::predicate_pushdown::PredicatePushdown::default()),
// Lifts the information `!isnull(col)`
Box::new(crate::nonnullable::NonNullable),
// Lifts the information `col = literal`
// TODO (#6613): this also tries to lift `!isnull(col)` but
// less well than the previous transform. Eliminate
// redundancy between the two transforms.
Box::new(crate::column_knowledge::ColumnKnowledge::default()),
// Lifts the information `col1 = col2`
Box::new(crate::demand::Demand::default()),
Box::new(crate::FuseAndCollapse::default()),
],
}),
// 4. Reduce/Join simplifications.
Box::new(crate::Fixpoint {
limit: 100,
transforms: vec![
// Pushes aggregations down
Box::new(crate::reduction_pushdown::ReductionPushdown),
// Replaces reduces with maps when the group keys are
// unique with maps
Box::new(crate::reduce_elision::ReduceElision),
// Converts `Cross Join {Constant(Literal) + Input}` to
// `Map {Cross Join (Input, Constant()), Literal}`.
// Join fusion will clean this up to `Map{Input, Literal}`
Box::new(crate::map_lifting::LiteralLifting::default()),
// Identifies common relation subexpressions.
// Must be followed by let inlining, to keep under control.
Box::new(crate::cse::relation_cse::RelationCSE),
Box::new(crate::inline_let::InlineLet::new(false)),
Box::new(crate::update_let::UpdateLet::default()),
Box::new(crate::FuseAndCollapse::default()),
],
}),
];
Self { transforms }
}
/// Builds a physical optimizer.
///
/// Performs logical transformations followed by all physical ones.
/// This is meant to be used for optimizing each view within a dataflow
/// once view inlining has already happened, right before dataflow
/// rendering.
pub fn physical_optimizer() -> Self {
// Implementation transformations
let transforms: Vec<Box<dyn crate::Transform>> = vec![
Box::new(crate::Fixpoint {
limit: 100,
transforms: vec![
Box::new(crate::join_implementation::JoinImplementation::default()),
Box::new(crate::column_knowledge::ColumnKnowledge::default()),
Box::new(crate::reduction::FoldConstants { limit: Some(10000) }),
Box::new(crate::demand::Demand::default()),
Box::new(crate::map_lifting::LiteralLifting::default()),
],
}),
Box::new(crate::canonicalize_mfp::CanonicalizeMfp),
// Identifies common relation subexpressions.
// Must be followed by let inlining, to keep under control.
Box::new(crate::cse::relation_cse::RelationCSE),
Box::new(crate::inline_let::InlineLet::new(false)),
Box::new(crate::update_let::UpdateLet::default()),
Box::new(crate::reduction::FoldConstants { limit: Some(10000) }),
];
Self { transforms }
}
/// Contains the logical optimizations that should run after cross-view
/// transformations run.
pub fn logical_cleanup_pass() -> Self {
let transforms: Vec<Box<dyn crate::Transform>> = vec![
// Delete unnecessary maps.
Box::new(crate::fusion::map::Map),
Box::new(crate::Fixpoint {
limit: 100,
transforms: vec![
// Projection pushdown may unblock fusing joins and unions.
Box::new(crate::fusion::join::Join),
Box::new(crate::redundant_join::RedundantJoin::default()),
// Redundant join produces projects that need to be fused.
Box::new(crate::fusion::project::Project),
Box::new(crate::fusion::union::Union),
// This goes after union fusion so we can cancel out
// more branches at a time.
Box::new(crate::union_cancel::UnionBranchCancellation),
Box::new(crate::cse::relation_cse::RelationCSE),
Box::new(crate::inline_let::InlineLet::new(true)),
Box::new(crate::reduction::FoldConstants { limit: Some(10000) }),
],
}),
];
Self { transforms }
}
/// Optimizes the supplied relation expression.
///
/// These optimizations are performed with no information about available arrangements,
/// which makes them suitable for pre-optimization before dataflow deployment.
pub fn optimize(
&mut self,
mut relation: MirRelationExpr,
) -> Result<expr::OptimizedMirRelationExpr, TransformError> {
self.transform(&mut relation, &HashMap::new())?;
Ok(expr::OptimizedMirRelationExpr(relation))
}
/// Optimizes the supplied relation expression in place, using available arrangements.
///
/// This method should only be called with non-empty `indexes` when optimizing a dataflow,
/// as the optimizations may lock in the use of arrangements that may cease to exist.
fn transform(
&self,
relation: &mut MirRelationExpr,
indexes: &HashMap<GlobalId, Vec<(GlobalId, Vec<MirScalarExpr>)>>,
) -> Result<(), TransformError> {
let mut id_gen = Default::default();
for transform in self.transforms.iter() {
transform.transform(
relation,
TransformArgs {
id_gen: &mut id_gen,
indexes,
},
)?;
}
Ok(())
}
}