query-engine/src/execution/mod.rs

//! Minimal execution support for the first SQL slice.

use std::error::Error;
use std::fmt;

use crate::chase::{Instance, Term};
use crate::planner::logical::{LogicalExpr, LogicalPlan};
use crate::relational::{ResultSet, Row, Value};

/// Errors returned by the current logical-plan executor.
#[derive(Debug)]
pub enum ExecutionError {
    /// A column reference could not be resolved.
    UnknownColumn(String),
    /// The scan layer encountered a variable term where a ground value was expected.
    NonGroundScanTerm,
}

impl fmt::Display for ExecutionError {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        match self {
            Self::UnknownColumn(column) => write!(f, "unknown column `{}`", column),
            Self::NonGroundScanTerm => {
                write!(f, "cannot scan non-ground terms into relational rows")
            }
        }
    }
}

impl Error for ExecutionError {}

/// Execute the current logical-plan subset against an instance-backed source.
pub fn execute(plan: &LogicalPlan, instance: &Instance) -> Result<ResultSet, ExecutionError> {
    match plan {
        LogicalPlan::Scan { table, schema } => {
            let mut rows = Vec::new();
            for fact in instance.facts_for_predicate(table) {
                let values = fact
                    .terms
                    .iter()
                    .map(value_from_term)
                    .collect::<Result<Vec<_>, _>>()?;
                rows.push(Row::new(values));
            }
            Ok(ResultSet::new(schema.clone(), rows))
        }
        LogicalPlan::Filter { input, predicate } => {
            let result = execute(input, instance)?;
            let filtered_rows = result
                .rows()
                .iter()
                .filter(|row| eval_predicate(predicate, row, result.schema()).unwrap_or(false))
                .cloned()
                .collect();
            Ok(ResultSet::new(result.schema().clone(), filtered_rows))
        }
        LogicalPlan::Project {
            input,
            expressions,
            schema,
        } => {
            let result = execute(input, instance)?;
            let mut rows = Vec::new();
            for row in result.rows() {
                let values = expressions
                    .iter()
                    .map(|expr| eval_expr(&expr.expr, row, result.schema()))
                    .collect::<Result<Vec<_>, _>>()?;
                rows.push(Row::new(values));
            }
            Ok(ResultSet::new(schema.clone(), rows))
        }
    }
}

fn eval_predicate(
    expr: &LogicalExpr,
    row: &Row,
    schema: &crate::relational::Schema,
) -> Result<bool, ExecutionError> {
    match expr {
        LogicalExpr::Eq(left, right) => Ok(eval_expr(left, row, schema)?
            .sql_eq(&eval_expr(right, row, schema)?)
            .unwrap_or(false)),
        _ => Ok(false),
    }
}

fn eval_expr(
    expr: &LogicalExpr,
    row: &Row,
    schema: &crate::relational::Schema,
) -> Result<Value, ExecutionError> {
    match expr {
        LogicalExpr::Column(name) => {
            let index = schema
                .index_of(name)
                .ok_or_else(|| ExecutionError::UnknownColumn(name.clone()))?;
            Ok(row.get(index).cloned().unwrap_or(Value::Null))
        }
        LogicalExpr::Literal(value) => Ok(value.clone()),
        LogicalExpr::Eq(left, right) => {
            let left = eval_expr(left, row, schema)?;
            let right = eval_expr(right, row, schema)?;
            Ok(Value::Boolean(left.sql_eq(&right).unwrap_or(false)))
        }
    }
}

fn value_from_term(term: &Term) -> Result<Value, ExecutionError> {
    match term {
        Term::Constant(value) => Ok(Value::text(value.clone())),
        Term::Null(_) => Ok(Value::Null),
        Term::Variable(_) => Err(ExecutionError::NonGroundScanTerm),
    }
}