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//! Rule-based optimizer for `LogicalPlan`.
//!
//! The optimizer applies a set of rewrite rules bottom-up until no rule fires
//! (fixed-point iteration). Each rule receives ownership of a node and returns
//! either a rewritten plan (`Ok`) or the original plan unchanged (`Err`).
use crate::ir::logical::LogicalPlan;
/// A single rewrite rule applied to a `LogicalPlan` node.
pub(crate) trait Rule: Send + Sync {
/// Try to rewrite `plan`. Return `Ok(new_plan)` if the rule fired; `Err(unchanged)` if not.
fn apply(&self, plan: LogicalPlan) -> Result<LogicalPlan, LogicalPlan>;
}
/// Applies a set of rules to a `LogicalPlan` until no rule fires (fixed-point).
pub(crate) struct Optimizer {
rules: Vec<Box<dyn Rule>>,
}
impl Optimizer {
pub(crate) fn new(rules: Vec<Box<dyn Rule>>) -> Self {
Self { rules }
}
/// Returns a default optimizer with the standard rule set.
pub(crate) fn default_rules() -> Self {
Self::new(vec![
Box::new(rules::StrengthReduce),
Box::new(rules::RedundantOps),
Box::new(rules::FilterBeforeMap),
])
}
/// Optimizes `plan` bottom-up; repeats until no rule fires.
pub(crate) fn optimize(&self, plan: LogicalPlan) -> LogicalPlan {
self.optimize_node(plan)
}
fn optimize_node(&self, plan: LogicalPlan) -> LogicalPlan {
// 1. Recurse into children first (bottom-up)
let plan = self.optimize_children(plan);
// 2. Apply rules at this node until fixed point
self.apply_rules(plan)
}
fn optimize_children(&self, plan: LogicalPlan) -> LogicalPlan {
match plan.take_input() {
Ok((child, shell)) => {
let optimized_child = self.optimize_node(*child);
shell.with_input(optimized_child)
}
Err(leaf) => leaf,
}
}
fn apply_rules(&self, mut plan: LogicalPlan) -> LogicalPlan {
let mut changed = true;
while changed {
changed = false;
for rule in &self.rules {
match rule.apply(plan) {
Ok(new_plan) => {
plan = new_plan;
changed = true;
}
Err(unchanged) => {
plan = unchanged;
}
}
}
}
plan
}
}
pub(crate) mod rules {
use super::Rule;
use crate::ir::logical::LogicalPlan;
/// Replaces expensive patterns with cheaper equivalents:
/// - `Sort(asc, no key) → Take(1)` → `Min`
/// - `Sort(desc, no key) → Take(1)` → `Max`
/// - `Sort(asc, no key) → First` → `Min`
/// - `Sort(desc, no key) → First` → `Max`
pub(crate) struct StrengthReduce;
impl Rule for StrengthReduce {
fn apply(&self, plan: LogicalPlan) -> Result<LogicalPlan, LogicalPlan> {
// Match Take(1, Sort(...)) or First(Sort(...))
// Track which outer form we had so we can rebuild correctly on Err.
enum Outer { Take1, First }
let (outer, inner) = match plan {
LogicalPlan::Take { n: 1, input } => (Outer::Take1, input),
LogicalPlan::First(input) => (Outer::First, input),
other => return Err(other),
};
match *inner {
LogicalPlan::Sort { input: sort_input, spec } => {
// Only collapse when there is no key expression (identity sort).
// A keyed sort changes semantics — we cannot simply emit Min/Max
// which operate on the raw value, not the key projection.
if spec.key.is_none() {
if spec.descending {
Ok(LogicalPlan::Max(sort_input))
} else {
Ok(LogicalPlan::Min(sort_input))
}
} else {
// Cannot collapse — rebuild the original outer node with sort inside.
let rebuilt_sort = LogicalPlan::Sort { input: sort_input, spec };
let rebuilt = match outer {
Outer::Take1 => LogicalPlan::Take {
n: 1,
input: Box::new(rebuilt_sort),
},
Outer::First => LogicalPlan::First(Box::new(rebuilt_sort)),
};
Err(rebuilt)
}
}
other => {
// Inner was not a Sort — rebuild the original outer node.
let rebuilt = match outer {
Outer::Take1 => LogicalPlan::Take {
n: 1,
input: Box::new(other),
},
Outer::First => LogicalPlan::First(Box::new(other)),
};
Err(rebuilt)
}
}
}
}
/// Cancels algebraically inverse operations:
/// - `Reverse(Reverse(x))` → `x`
/// - `Take(n1)(Take(n2)(x))` → `Take(min(n1, n2))(x)`
/// - `Skip(0)(x)` → `x`
pub(crate) struct RedundantOps;
impl Rule for RedundantOps {
fn apply(&self, plan: LogicalPlan) -> Result<LogicalPlan, LogicalPlan> {
match plan {
LogicalPlan::Reverse { input } => {
if let LogicalPlan::Reverse { input: inner } = *input {
Ok(*inner)
} else {
Err(LogicalPlan::Reverse { input })
}
}
LogicalPlan::Take { n: n1, input } => {
if let LogicalPlan::Take { n: n2, input: inner } = *input {
Ok(LogicalPlan::Take {
n: n1.min(n2),
input: inner,
})
} else {
Err(LogicalPlan::Take { n: n1, input })
}
}
LogicalPlan::Skip { n: 0, input } => Ok(*input),
_ => Err(plan),
}
}
}
/// Moves `Filter` before `Map` when the predicate does not reference
/// the map's output. This reduces rows that need to be mapped.
pub(crate) struct FilterBeforeMap;
impl Rule for FilterBeforeMap {
fn apply(&self, plan: LogicalPlan) -> Result<LogicalPlan, LogicalPlan> {
// Filter { input: Map { input: x, projection: f }, predicate: p }
// → Map { input: Filter { input: x, predicate: p }, projection: f }
// only when p does not reference the map output
match plan {
LogicalPlan::Filter { input, predicate }
if matches!(*input, LogicalPlan::Map { .. }) =>
{
if let LogicalPlan::Map {
input: map_input,
projection,
} = *input
{
if is_independent_predicate(&predicate) {
return Ok(LogicalPlan::Map {
input: Box::new(LogicalPlan::Filter {
input: map_input,
predicate,
}),
projection,
});
}
return Err(LogicalPlan::Filter {
input: Box::new(LogicalPlan::Map {
input: map_input,
projection,
}),
predicate,
});
}
unreachable!()
}
_ => Err(plan),
}
}
}
/// Returns `true` when `expr` can be safely moved before a `Map` stage —
/// it accesses only original source fields, not computed map outputs.
///
/// Conservative: only allow literals, root/current references, simple field
/// chains, and binary/unary combinations of the above.
fn is_independent_predicate(expr: &crate::parse::ast::Expr) -> bool {
use crate::parse::ast::Expr;
match expr {
// Literals — always safe
Expr::Null | Expr::Bool(_) | Expr::Int(_) | Expr::Float(_) | Expr::Str(_) => true,
// Document root and current-item reference — safe to move
Expr::Root | Expr::Current => true,
// Plain identifier that resolves against the current row — safe
Expr::Ident(_) => true,
// Simple field navigation chains (e.g. `@.price`)
Expr::Chain(base, steps) => {
use crate::parse::ast::Step;
let base_ok = is_independent_predicate(base);
let steps_ok = steps.iter().all(|s| matches!(s, Step::Field(_) | Step::Index(_)));
base_ok && steps_ok
}
// Compound expressions — recurse
Expr::BinOp(lhs, _, rhs) => {
is_independent_predicate(lhs) && is_independent_predicate(rhs)
}
Expr::Not(inner) => is_independent_predicate(inner),
Expr::UnaryNeg(inner) => is_independent_predicate(inner),
Expr::Coalesce(lhs, rhs) => {
is_independent_predicate(lhs) && is_independent_predicate(rhs)
}
// Anything with method calls, lambdas, let-bindings, comprehensions — conservatively false
_ => false,
}
}
}