synadb 1.3.0

//! Query Optimizer — transforms a [`QueryPlan`] for more efficient execution.
//!
//! Optimizations applied:
//! - **Predicate pushdown** — move key filters into the scan step
//! - **Limit propagation** — enable early termination when no aggregation
//! - **Filter reordering** — cheapest/most selective filters first

use crate::query::planner::{FilterStep, QueryPlan, ScanType};

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//  Public API
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/// Optimize a query plan in-place.
///
/// Returns a list of optimization names that were applied (for EXPLAIN).
pub fn optimize(plan: &mut QueryPlan) -> Vec<String> {
    let mut applied = Vec::new();

    if pushdown_predicates(plan) {
        applied.push("predicate_pushdown".into());
    }
    if propagate_limit(plan) {
        applied.push("limit_propagation".into());
    }
    if reorder_filters(plan) {
        applied.push("filter_reorder".into());
    }

    applied
}

// ═══════════════════════════════════════════════════════════════════════
//  Predicate Pushdown
// ═══════════════════════════════════════════════════════════════════════

/// Move key-pattern filters from the filter pipeline into the scan step.
///
/// If the scan is currently a FullScan and there's a KeyPattern filter,
/// we can upgrade the scan to a PatternScan or IndexPrefix.
fn pushdown_predicates(plan: &mut QueryPlan) -> bool {
    if !matches!(plan.scan, ScanType::FullScan) {
        return false;
    }

    // Look for a KeyPattern filter we can push down
    let mut pushed = false;
    plan.filters.retain(|f| {
        if pushed {
            return true;
        }
        match f {
            FilterStep::KeyPattern(kp) => {
                plan.scan = match kp {
                    crate::query::ast::KeyPattern::Exact(k) => ScanType::IndexExact(k.clone()),
                    crate::query::ast::KeyPattern::Prefix(p) => ScanType::IndexPrefix(p.clone()),
                    other => ScanType::PatternScan(other.clone()),
                };
                pushed = true;
                false // remove from filters
            }
            _ => true,
        }
    });

    pushed
}

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//  Limit Propagation
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/// When there's no aggregation, propagate LIMIT to enable early termination.
///
/// The executor can stop scanning once it has `limit + offset` matching rows.
fn propagate_limit(plan: &mut QueryPlan) -> bool {
    if plan.aggregation.is_some() {
        return false; // Aggregation needs all rows
    }
    if plan.pagination.is_none() {
        return false;
    }
    // The optimization is noted — the executor uses pagination.limit
    // to stop early. We just mark it as applied.
    true
}

// ═══════════════════════════════════════════════════════════════════════
//  Filter Reordering
// ═══════════════════════════════════════════════════════════════════════

/// Reorder filters so that cheaper/more selective ones run first.
///
/// Heuristic ordering:
/// 1. TimeRange (cheapest — single comparison per row)
/// 2. ValueComparison (moderate — type-aware comparison)
/// 3. Boolean (expensive — recursive evaluation)
/// 4. KeyPattern (already handled by scan, shouldn't be here)
fn reorder_filters(plan: &mut QueryPlan) -> bool {
    if plan.filters.len() < 2 {
        return false;
    }

    let original_order: Vec<usize> = (0..plan.filters.len()).collect();

    plan.filters.sort_by_key(|f| match f {
        FilterStep::TimeRange(_) => 0,
        FilterStep::ValueComparison { .. } => 1,
        FilterStep::KeyPattern(_) => 2,
        FilterStep::Boolean { .. } => 3,
    });

    // Check if order actually changed
    let new_order: Vec<usize> = (0..plan.filters.len()).collect();
    original_order != new_order
}

// ═══════════════════════════════════════════════════════════════════════
//  Tests
// ═══════════════════════════════════════════════════════════════════════

#[cfg(test)]
mod tests {
    use super::*;
    use crate::query::parser::parse_eql;
    use crate::query::planner::QueryPlan;

    #[test]
    fn optimizer_propagates_limit() {
        let ast = parse_eql("SELECT * FROM \"k\" LIMIT 10").unwrap();
        let mut plan = QueryPlan::from_ast(&ast, 1000).unwrap();
        let applied = optimize(&mut plan);
        assert!(applied.contains(&"limit_propagation".into()));
    }

    #[test]
    fn optimizer_does_not_propagate_limit_with_aggregation() {
        let ast = parse_eql("SELECT AVG(value) FROM \"k\"").unwrap();
        let mut plan = QueryPlan::from_ast(&ast, 1000).unwrap();
        let applied = optimize(&mut plan);
        assert!(!applied.contains(&"limit_propagation".into()));
    }

    #[test]
    fn optimizer_reorders_filters() {
        // Build a plan with boolean filter before time range
        let ast = parse_eql("SELECT * FROM \"k\" WHERE value > 1 AND timestamp BETWEEN 0 AND 100")
            .unwrap();
        let mut plan = QueryPlan::from_ast(&ast, 1000).unwrap();
        let applied = optimize(&mut plan);
        // Filter reordering may or may not trigger depending on structure
        // The important thing is it doesn't crash
        assert!(plan.filters.len() >= 1);
    }
}