kotoba_query_engine/

optimizer.rs

//! Query Optimizer
//!
//! Optimizes GQL queries for efficient execution.

use std::sync::Arc;
use async_trait::async_trait;
use anyhow::Result;

use crate::ast::*;
use crate::types::*;
use crate::IndexManagerPort;
use kotoba_storage::KeyValueStore;

/// Query optimizer with KeyValueStore backend
pub struct QueryOptimizer<T: KeyValueStore> {
    storage: Arc<T>,
}

impl<T: KeyValueStore + 'static> QueryOptimizer<T> {
    pub fn new(storage: Arc<T>) -> Self {
        Self { storage }
    }

    /// Optimize a parsed query
    pub async fn optimize(&self, mut query: GqlQuery) -> Result<GqlQuery> {
        // Apply various optimization rules
        query = self.reorder_clauses(query).await?;
        query = self.push_down_filters(query).await?;
        query = self.optimize_joins(query).await?;
        query = self.select_indices(query).await?;

        Ok(query)
    }

    /// Reorder query clauses for better performance
    async fn reorder_clauses(&self, mut query: GqlQuery) -> Result<GqlQuery> {
        // Reorder clauses to execute filters as early as possible
        let mut match_clauses = Vec::new();
        let mut filter_clauses = Vec::new();
        let mut other_clauses = Vec::new();

        for clause in query.clauses {
            match clause {
                QueryClause::Match(_) => match_clauses.push(clause),
                QueryClause::Where(_) => filter_clauses.push(clause),
                _ => other_clauses.push(clause),
            }
        }

        // Put filters right after match clauses for early filtering
        let mut reordered = Vec::new();
        reordered.extend(match_clauses);
        reordered.extend(filter_clauses);
        reordered.extend(other_clauses);

        query.clauses = reordered;
        Ok(query)
    }

    /// Push down filters to data source level
    async fn push_down_filters(&self, mut query: GqlQuery) -> Result<GqlQuery> {
        // Analyze WHERE clauses and push them down to MATCH clauses where possible
        let mut where_clauses = Vec::new();
        let mut new_clauses = Vec::new();

        for clause in query.clauses {
            match clause {
                QueryClause::Where(where_clause) => {
                    where_clauses.push(where_clause);
                }
                QueryClause::Match(mut match_clause) => {
                    // Try to push down filters to the match clause
                    if let Some(filter) = where_clauses.pop() {
                        if self.can_push_filter(&match_clause, &filter) {
                            // TODO: Modify match_clause to include the filter
                            new_clauses.push(QueryClause::Match(match_clause));
                        } else {
                            new_clauses.push(QueryClause::Match(match_clause));
                            new_clauses.push(QueryClause::Where(filter));
                        }
                    } else {
                        new_clauses.push(QueryClause::Match(match_clause));
                    }
                }
                _ => new_clauses.push(clause),
            }
        }

        // Add remaining WHERE clauses
        for where_clause in where_clauses {
            new_clauses.push(QueryClause::Where(where_clause));
        }

        query.clauses = new_clauses;
        Ok(query)
    }

    /// Optimize join order and strategy
    async fn optimize_joins(&self, query: GqlQuery) -> Result<GqlQuery> {
        // TODO: Implement join optimization
        // For now, just return the query as-is
        Ok(query)
    }

    /// Select appropriate indices for query execution
    async fn select_indices(&self, query: GqlQuery) -> Result<GqlQuery> {
        // TODO: Analyze query and suggest index usage
        Ok(query)
    }

    /// Check if a filter can be pushed down to a MATCH clause
    fn can_push_filter(&self, _match_clause: &MatchClause, _filter: &WhereClause) -> bool {
        // TODO: Implement filter pushdown analysis
        // For now, assume filters cannot be pushed down
        false
    }
}

/// Cost estimation for query optimization
pub struct CostEstimator<T: KeyValueStore> {
    storage: Arc<T>,
}

impl<T: KeyValueStore + 'static> CostEstimator<T> {
    pub fn new(storage: Arc<T>) -> Self {
        Self { storage }
    }

    /// Estimate the cost of executing a query plan
    pub async fn estimate_cost(&self, _plan: &crate::planner::ExecutionPlan) -> Result<QueryCost> {
        // TODO: Implement cost estimation based on storage characteristics
        Ok(QueryCost {
            cpu_cost: 1.0,
            io_cost: 1.0,
            network_cost: 0.0,
            total_cost: 2.0,
        })
    }
}

/// Query execution cost
#[derive(Debug, Clone)]
pub struct QueryCost {
    pub cpu_cost: f64,
    pub io_cost: f64,
    pub network_cost: f64,
    pub total_cost: f64,
}

/// Optimization rules
pub mod rules {
    use super::*;

    /// Rule for pushing down filters
    pub struct FilterPushdownRule;

    impl FilterPushdownRule {
        pub async fn apply(query: GqlQuery) -> Result<GqlQuery> {
            // TODO: Implement filter pushdown logic
            Ok(query)
        }
    }

    /// Rule for reordering joins
    pub struct JoinReorderRule;

    impl JoinReorderRule {
        pub async fn apply(query: GqlQuery) -> Result<GqlQuery> {
            // TODO: Implement join reordering logic
            Ok(query)
        }
    }

    /// Rule for selecting indices
    pub struct IndexSelectionRule;

    impl IndexSelectionRule {
        pub async fn apply(query: GqlQuery) -> Result<GqlQuery> {
            // TODO: Implement index selection logic
            Ok(query)
        }
    }
}

/// Statistics for optimization
#[async_trait]
pub trait StatisticsProvider: Send + Sync {
    /// Get the selectivity of a filter
    async fn get_selectivity(&self, filter: &BooleanExpression) -> Result<f64>;

    /// Get the cardinality of a vertex pattern
    async fn get_vertex_cardinality(&self, pattern: &VertexPattern) -> Result<u64>;

    /// Get the cardinality of an edge pattern
    async fn get_edge_cardinality(&self, pattern: &EdgePattern) -> Result<u64>;
}

/// Statistics implementation with KeyValueStore backend
pub struct StatisticsManager<T: KeyValueStore> {
    storage: Arc<T>,
}

impl<T: KeyValueStore + 'static> StatisticsManager<T> {
    pub fn new(storage: Arc<T>) -> Self {
        Self { storage }
    }
}

#[async_trait]
impl<T: KeyValueStore + 'static> StatisticsProvider for StatisticsManager<T> {
    async fn get_selectivity(&self, _filter: &BooleanExpression) -> Result<f64> {
        // TODO: Implement selectivity estimation based on storage statistics
        Ok(0.1) // Placeholder: assume 10% selectivity
    }

    async fn get_vertex_cardinality(&self, _pattern: &VertexPattern) -> Result<u64> {
        // TODO: Implement cardinality estimation based on storage data
        Ok(1000) // Placeholder
    }

    async fn get_edge_cardinality(&self, _pattern: &EdgePattern) -> Result<u64> {
        // TODO: Implement cardinality estimation based on storage data
        Ok(5000) // Placeholder
    }
}

#[cfg(test)]
mod tests {
    use super::*;

    #[test]
    fn test_query_optimizer_creation() {
        // Test that optimizer can be created
        // This will be expanded with actual optimization tests
    }
}