oxgraph_db/

query.rs

//! Native `OxQL` query execution.

use serde::{Deserialize, Serialize};

use crate::{
    DbError, ElementId, IncidenceRecord, ProjectionId, PropertyKeyId, PropertySubject,
    PropertyValue, RelationId,
    catalog::{ProjectionDefinition, PropertyFamily},
    overlay::StateView,
    projection::GraphProjection,
    traversal::{self, Direction, Walk},
    value::parse_value_token,
};

/// Prepared query plan.
///
/// # Performance
///
/// Cloning is `O(query length)`.
#[derive(Clone, Debug, Eq, PartialEq)]
pub struct PreparedQuery {
    /// Original query text.
    text: String,
    /// Physical plan.
    plan: QueryPlan,
}

impl PreparedQuery {
    /// Parses and prepares one query string against current catalog metadata.
    ///
    /// # Errors
    ///
    /// Returns [`DbError::EmptyQuery`] or [`DbError::UnsupportedQuery`] when the
    /// query is outside the supported profile.
    ///
    /// # Performance
    ///
    /// This function is `O(query length + catalog lookup cost)`.
    pub(crate) fn prepare(query: &str, state: &impl StateView) -> Result<Self, DbError> {
        let trimmed = query.trim();
        if trimmed.is_empty() {
            return Err(DbError::EmptyQuery);
        }
        let logical = parse_oxql(trimmed)?;
        let plan = bind_and_lower(logical, state)?;
        Ok(Self {
            text: trimmed.to_owned(),
            plan,
        })
    }

    /// Returns the source query text.
    ///
    /// # Performance
    ///
    /// This method is `O(1)`.
    #[must_use]
    pub fn text(&self) -> &str {
        &self.text
    }

    /// Returns a stable physical explanation.
    ///
    /// # Performance
    ///
    /// This method is `O(plan size)`.
    #[must_use]
    pub fn explain(&self) -> String {
        self.plan.explain()
    }

    /// Executes this prepared plan.
    ///
    /// # Errors
    ///
    /// Returns [`DbError`] when a referenced projection cannot be materialized.
    ///
    /// # Performance
    ///
    /// This method is `O(scanned rows × predicate size + plan output +
    /// projection build cost when used)`.
    pub(crate) fn execute(&self, state: &impl StateView) -> Result<QueryResult, DbError> {
        self.plan.execute(state)
    }
}

/// Query result materialized for JSON, CLI, and embedded consumers.
///
/// # Performance
///
/// Iterating rows is `O(row count)`.
#[derive(Clone, Debug, Deserialize, Eq, PartialEq, Serialize)]
pub struct QueryResult {
    /// Materialized rows.
    rows: Vec<QueryRow>,
}

impl QueryResult {
    /// Creates a result from rows.
    ///
    /// # Performance
    ///
    /// This function is `O(1)` excluding row ownership transfer.
    #[must_use]
    pub(crate) const fn new(rows: Vec<QueryRow>) -> Self {
        Self { rows }
    }

    /// Returns result rows.
    ///
    /// # Performance
    ///
    /// This method is `O(1)`.
    #[must_use]
    pub fn rows(&self) -> &[QueryRow] {
        &self.rows
    }
}

/// One query result row.
///
/// # Performance
///
/// Cloning is `O(value count + string/property bytes)`.
#[derive(Clone, Debug, Deserialize, Eq, PartialEq, Serialize)]
pub struct QueryRow {
    /// Values carried by the row.
    pub values: Vec<QueryValue>,
}

impl QueryRow {
    /// Creates a single-value row.
    ///
    /// # Performance
    ///
    /// This function is `O(1)` excluding value ownership transfer.
    #[must_use]
    pub(crate) fn single(value: QueryValue) -> Self {
        Self {
            values: vec![value],
        }
    }
}

/// Query value variants used by `OxQL`.
///
/// # Performance
///
/// Cloning is `O(value bytes)`.
#[derive(Clone, Debug, Deserialize, Eq, PartialEq, Serialize)]
pub enum QueryValue {
    /// Canonical element id.
    Element(ElementId),
    /// Canonical relation id.
    Relation(RelationId),
    /// Canonical incidence record.
    Incidence(IncidenceRecord),
    /// Property subject.
    Subject(PropertySubject),
    /// Typed property value.
    Property(PropertyValue),
    /// Human-readable text value.
    Text(String),
    /// Projection id.
    Projection(ProjectionId),
}

/// Private physical query plan.
#[derive(Clone, Debug, Eq, PartialEq)]
enum QueryPlan {
    /// Scan all elements.
    ElementScan,
    /// Scan all relations.
    RelationScan,
    /// Scan all incidences.
    IncidenceScan,
    /// Scan elements with a label.
    ElementLabelScan {
        /// Label name for explanations.
        label: String,
        /// Label ID.
        label_id: crate::LabelId,
    },
    /// Scan elements by property equality.
    ElementPropertyEqual {
        /// Property key name for explanations.
        key: String,
        /// Property key ID.
        key_id: PropertyKeyId,
        /// Required property value.
        value: PropertyValue,
    },
    /// Filter elements by a bound compound property predicate.
    ElementFilter {
        /// Bound predicate tree.
        predicate: BoundPredicate,
    },
    /// Scan relations by relation type.
    RelationTypeScan {
        /// Relation type name for explanations.
        relation_type: String,
        /// Relation type ID.
        relation_type_id: crate::RelationTypeId,
    },
    /// Walk a graph projection.
    GraphWalk {
        /// Projection ID.
        projection: ProjectionId,
        /// Canonical starting element.
        element: ElementId,
        /// Traversal options.
        options: Walk,
    },
    /// Catalog metadata scan.
    CatalogScan,
}

/// Bound element predicate produced by lowering a [`LogicalPredicate`].
#[derive(Clone, Debug, Eq, PartialEq)]
enum BoundPredicate {
    /// `key <op> value` against a resolved key and typed value.
    Compare {
        /// Property key name for explanations.
        key: String,
        /// Resolved property key id.
        key_id: PropertyKeyId,
        /// Comparison operator.
        op: CompareOp,
        /// Typed comparison value.
        value: PropertyValue,
    },
    /// Conjunction of two predicates.
    And(Box<Self>, Box<Self>),
    /// Disjunction of two predicates.
    Or(Box<Self>, Box<Self>),
}

impl BoundPredicate {
    /// Formats this predicate for plan explanations.
    fn explain(&self) -> String {
        match self {
            Self::Compare { key, op, value, .. } => format!("{key} {} {value}", op.spelling()),
            Self::And(left, right) => format!("({} AND {})", left.explain(), right.explain()),
            Self::Or(left, right) => format!("({} OR {})", left.explain(), right.explain()),
        }
    }

    /// Returns whether `element` satisfies this predicate in `state`.
    ///
    /// A missing property never satisfies a comparison.
    fn evaluate(&self, state: &impl StateView, element: ElementId) -> bool {
        match self {
            Self::Compare {
                key_id, op, value, ..
            } => state
                .property(PropertySubject::Element(element), *key_id)
                .is_some_and(|actual| op.matches(actual.as_ref().cmp(value))),
            Self::And(left, right) => {
                left.evaluate(state, element) && right.evaluate(state, element)
            }
            Self::Or(left, right) => {
                left.evaluate(state, element) || right.evaluate(state, element)
            }
        }
    }
}

impl QueryPlan {
    /// Explains this physical plan.
    fn explain(&self) -> String {
        match self {
            Self::ElementScan => "oxql scan elements".to_owned(),
            Self::RelationScan => "oxql scan relations".to_owned(),
            Self::IncidenceScan => "oxql scan incidences".to_owned(),
            Self::ElementLabelScan { label, .. } => {
                format!("oxql label index lookup elements label={label}")
            }
            Self::ElementPropertyEqual { key, value, .. } => {
                format!("oxql property equality lookup elements {key}={value}")
            }
            Self::ElementFilter { predicate } => {
                format!("oxql element filter {}", predicate.explain())
            }
            Self::RelationTypeScan { relation_type, .. } => {
                format!("oxql relation-type index lookup type={relation_type}")
            }
            Self::GraphWalk {
                projection,
                element,
                options,
            } => format!(
                "oxql graph projection {projection} walk from {element} depth {} direction {:?} limit {}",
                options.max_depth, options.direction, options.limit,
            ),
            Self::CatalogScan => "catalog metadata scan".to_owned(),
        }
    }

    /// Executes this physical plan.
    fn execute(&self, state: &impl StateView) -> Result<QueryResult, DbError> {
        match self {
            Self::ElementScan => Ok(scan_elements(state)),
            Self::RelationScan => Ok(scan_relations(state)),
            Self::IncidenceScan => Ok(scan_incidences(state)),
            Self::ElementLabelScan { label_id, .. } => {
                Ok(scan_elements_with_label(state, *label_id))
            }
            Self::ElementPropertyEqual { key_id, value, .. } => {
                scan_elements_with_property(state, *key_id, value)
            }
            Self::ElementFilter { predicate } => Ok(filter_elements(state, predicate)),
            Self::RelationTypeScan {
                relation_type_id, ..
            } => Ok(scan_relations_with_type(state, *relation_type_id)),
            Self::GraphWalk {
                projection,
                element,
                options,
            } => execute_graph_walk(state, *projection, *element, *options),
            Self::CatalogScan => Ok(scan_catalog(state)),
        }
    }
}

/// Resolved logical operation emitted by parsing, before catalog binding.
///
/// The parsers ([`parse_oxql`], [`parse_cypher`]) are purely token/string
/// driven and never touch the catalog; they produce one of these ops carrying
/// raw names and literals. [`bind_and_lower`] then resolves names to catalog
/// IDs, validates value families/types, and produces the physical
/// [`QueryPlan`].
#[derive(Clone, Debug, Eq, PartialEq)]
enum LogicalOp {
    /// Scan all elements.
    ElementScan,
    /// Scan all relations.
    RelationScan,
    /// Scan all incidences.
    IncidenceScan,
    /// Scan catalog metadata.
    CatalogScan,
    /// Scan elements carrying a named label.
    ElementLabelScan {
        /// Unresolved label name.
        label: String,
    },
    /// Scan relations of a named relation type.
    RelationTypeScan {
        /// Unresolved relation type name.
        relation_type: String,
    },
    /// Scan elements whose named property equals a literal token.
    ElementPropertyEqual {
        /// Unresolved property key name.
        key: String,
        /// Unparsed property value token.
        value: String,
    },
    /// Walk a named graph projection from a raw element-ID token.
    GraphWalk {
        /// Unresolved projection name.
        projection: String,
        /// Unparsed element-ID token.
        element: String,
        /// Parsed traversal options.
        options: Walk,
    },
    /// Filter elements by a compound property predicate.
    ElementWhere {
        /// Unbound predicate tree carrying raw key names and value tokens.
        predicate: LogicalPredicate,
    },
}

/// Comparison operator in an element property predicate.
#[derive(Clone, Copy, Debug, Eq, PartialEq)]
enum CompareOp {
    /// Equality (`=`).
    Eq,
    /// Strictly less than (`<`).
    Lt,
    /// Less than or equal (`<=`).
    Le,
    /// Strictly greater than (`>`).
    Gt,
    /// Greater than or equal (`>=`).
    Ge,
}

impl CompareOp {
    /// Returns whether `ordering` of the stored value against the literal
    /// satisfies this operator.
    const fn matches(self, ordering: core::cmp::Ordering) -> bool {
        use core::cmp::Ordering::{Equal, Greater, Less};
        match self {
            Self::Eq => matches!(ordering, Equal),
            Self::Lt => matches!(ordering, Less),
            Self::Le => matches!(ordering, Less | Equal),
            Self::Gt => matches!(ordering, Greater),
            Self::Ge => matches!(ordering, Greater | Equal),
        }
    }

    /// Returns the source spelling of this operator.
    const fn spelling(self) -> &'static str {
        match self {
            Self::Eq => "=",
            Self::Lt => "<",
            Self::Le => "<=",
            Self::Gt => ">",
            Self::Ge => ">=",
        }
    }
}

/// Raw element predicate parsed from a `WHERE` clause, before catalog binding.
///
/// Precedence is `OR` (loosest) over `AND` over comparisons; parentheses
/// override it. Leaves carry the raw key name and value token; binding resolves
/// them to catalog ids and typed values.
#[derive(Clone, Debug, Eq, PartialEq)]
enum LogicalPredicate {
    /// `key <op> value` comparison against a raw value token.
    Compare {
        /// Unresolved property key name.
        key: String,
        /// Comparison operator.
        op: CompareOp,
        /// Unparsed property value token.
        value: String,
    },
    /// Conjunction of two predicates.
    And(Box<Self>, Box<Self>),
    /// Disjunction of two predicates.
    Or(Box<Self>, Box<Self>),
}

/// Parses native `OxQL` into a logical operation without catalog access.
fn parse_oxql(query: &str) -> Result<LogicalOp, DbError> {
    let tokens = query.split_whitespace().collect::<Vec<_>>();
    let upper = tokens
        .iter()
        .map(|token| token.to_ascii_uppercase())
        .collect::<Vec<_>>();
    if upper.len() >= 3
        && upper[0].as_str() == "MATCH"
        && upper[1].as_str() == "ELEMENTS"
        && upper[2].as_str() == "WHERE"
    {
        return parse_element_where(&tokens[3..], &upper[3..]);
    }
    match upper.as_slice() {
        [command] if command == "CATALOG" => Ok(LogicalOp::CatalogScan),
        [verb, family] if verb == "MATCH" && family == "ELEMENTS" => Ok(LogicalOp::ElementScan),
        [verb, family] if verb == "MATCH" && family == "RELATIONS" => Ok(LogicalOp::RelationScan),
        [verb, family] if verb == "MATCH" && family == "INCIDENCES" => Ok(LogicalOp::IncidenceScan),
        [verb, family, has, object, _name]
            if verb == "MATCH" && family == "ELEMENTS" && has == "HAS" && object == "LABEL" =>
        {
            Ok(LogicalOp::ElementLabelScan {
                label: tokens[4].to_owned(),
            })
        }
        [verb, family, r#type, _name]
            if verb == "MATCH" && family == "RELATIONS" && r#type == "TYPE" =>
        {
            Ok(LogicalOp::RelationTypeScan {
                relation_type: tokens[3].to_owned(),
            })
        }
        [graph, _projection, neighbors, _element]
            if graph == "GRAPH" && neighbors == "NEIGHBORS" =>
        {
            Ok(LogicalOp::GraphWalk {
                projection: tokens[1].to_owned(),
                element: tokens[3].to_owned(),
                options: Walk::default(),
            })
        }
        [
            graph,
            _projection,
            walk,
            from,
            _element,
            depth,
            _max_depth,
            ..,
        ] if graph == "GRAPH" && walk == "WALK" && from == "FROM" && depth == "DEPTH" => {
            parse_graph_walk(&tokens, &upper)
        }
        _tokens => Err(DbError::unsupported("unsupported OxQL profile query")),
    }
}

/// Parses the optional `DIRECTION`/`LIMIT` clauses of an `OxQL` graph walk.
fn parse_graph_walk(tokens: &[&str], upper: &[String]) -> Result<LogicalOp, DbError> {
    let max_depth = tokens[6]
        .parse::<usize>()
        .map_err(|_error| DbError::unsupported("walk depth must be an integer"))?;
    let mut options = Walk {
        max_depth,
        ..Walk::default()
    };
    let mut saw_direction = false;
    let mut saw_limit = false;
    let mut index = 7;
    while index < tokens.len() {
        let Some(value) = tokens.get(index + 1) else {
            return Err(DbError::unsupported("walk option requires a value"));
        };
        match upper[index].as_str() {
            "DIRECTION" if !saw_direction => {
                options.direction = parse_walk_direction(&upper[index + 1])?;
                saw_direction = true;
            }
            "DIRECTION" => return Err(DbError::unsupported("walk direction specified twice")),
            "LIMIT" if !saw_limit => {
                options.limit = value
                    .parse::<usize>()
                    .map_err(|_error| DbError::unsupported("walk limit must be an integer"))?;
                saw_limit = true;
            }
            "LIMIT" => return Err(DbError::unsupported("walk limit specified twice")),
            _option => return Err(DbError::unsupported("unsupported walk option")),
        }
        index += 2;
    }
    Ok(LogicalOp::GraphWalk {
        projection: tokens[1].to_owned(),
        element: tokens[4].to_owned(),
        options,
    })
}

/// Parses one `OxQL` graph walk direction.
fn parse_walk_direction(direction: &str) -> Result<Direction, DbError> {
    match direction {
        "OUTGOING" => Ok(Direction::Outgoing),
        "INCOMING" => Ok(Direction::Incoming),
        "BOTH" => Ok(Direction::Both),
        _direction => Err(DbError::unsupported(
            "walk direction must be outgoing, incoming, or both",
        )),
    }
}

/// Parses an `OxQL` element `WHERE` predicate into a logical operation.
///
/// Collapses a lone `key = value` to the indexed
/// [`LogicalOp::ElementPropertyEqual`] fast path; any compound or ordered
/// predicate becomes [`LogicalOp::ElementWhere`].
fn parse_element_where(tokens: &[&str], upper: &[String]) -> Result<LogicalOp, DbError> {
    let mut cursor = 0;
    let predicate = parse_predicate_or(tokens, upper, &mut cursor)?;
    if cursor != tokens.len() {
        return Err(DbError::unsupported(
            "trailing tokens after WHERE predicate",
        ));
    }
    match predicate {
        LogicalPredicate::Compare {
            key,
            op: CompareOp::Eq,
            value,
        } => Ok(LogicalOp::ElementPropertyEqual { key, value }),
        predicate => Ok(LogicalOp::ElementWhere { predicate }),
    }
}

/// Parses a disjunction: `and ( OR and )*`.
fn parse_predicate_or(
    tokens: &[&str],
    upper: &[String],
    cursor: &mut usize,
) -> Result<LogicalPredicate, DbError> {
    let mut left = parse_predicate_and(tokens, upper, cursor)?;
    while upper.get(*cursor).map(String::as_str) == Some("OR") {
        *cursor += 1;
        let right = parse_predicate_and(tokens, upper, cursor)?;
        left = LogicalPredicate::Or(Box::new(left), Box::new(right));
    }
    Ok(left)
}

/// Parses a conjunction: `factor ( AND factor )*`.
fn parse_predicate_and(
    tokens: &[&str],
    upper: &[String],
    cursor: &mut usize,
) -> Result<LogicalPredicate, DbError> {
    let mut left = parse_predicate_factor(tokens, upper, cursor)?;
    while upper.get(*cursor).map(String::as_str) == Some("AND") {
        *cursor += 1;
        let right = parse_predicate_factor(tokens, upper, cursor)?;
        left = LogicalPredicate::And(Box::new(left), Box::new(right));
    }
    Ok(left)
}

/// Parses a parenthesized group or a single comparison.
fn parse_predicate_factor(
    tokens: &[&str],
    upper: &[String],
    cursor: &mut usize,
) -> Result<LogicalPredicate, DbError> {
    if tokens.get(*cursor) == Some(&"(") {
        *cursor += 1;
        let inner = parse_predicate_or(tokens, upper, cursor)?;
        if tokens.get(*cursor) != Some(&")") {
            return Err(DbError::unsupported(
                "unbalanced parentheses in WHERE predicate",
            ));
        }
        *cursor += 1;
        return Ok(inner);
    }
    parse_comparison(tokens, cursor)
}

/// Parses one `key <op> value` comparison.
fn parse_comparison(tokens: &[&str], cursor: &mut usize) -> Result<LogicalPredicate, DbError> {
    let key = tokens
        .get(*cursor)
        .ok_or_else(|| DbError::unsupported("expected property key in WHERE predicate"))?;
    let operator = tokens
        .get(*cursor + 1)
        .ok_or_else(|| DbError::unsupported("expected comparison operator in WHERE predicate"))?;
    let value = tokens
        .get(*cursor + 2)
        .ok_or_else(|| DbError::unsupported("expected value in WHERE predicate"))?;
    let op = parse_compare_op(operator)?;
    *cursor += 3;
    Ok(LogicalPredicate::Compare {
        key: (*key).to_owned(),
        op,
        value: (*value).to_owned(),
    })
}

/// Parses one comparison operator token.
fn parse_compare_op(token: &str) -> Result<CompareOp, DbError> {
    match token {
        "=" => Ok(CompareOp::Eq),
        "<" => Ok(CompareOp::Lt),
        "<=" => Ok(CompareOp::Le),
        ">" => Ok(CompareOp::Gt),
        ">=" => Ok(CompareOp::Ge),
        _operator => Err(DbError::unsupported(
            "comparison operator must be =, <, <=, >, or >=",
        )),
    }
}

/// Binds a raw element predicate against the catalog.
fn bind_predicate(
    predicate: LogicalPredicate,
    state: &impl StateView,
) -> Result<BoundPredicate, DbError> {
    match predicate {
        LogicalPredicate::Compare { key, op, value } => {
            let key_id = state
                .catalog()
                .property_key_id(&key)
                .ok_or_else(|| DbError::unsupported(format!("unknown property key {key}")))?;
            let value = parse_value_token(&value).map_err(DbError::unsupported)?;
            state.validate_lookup_value_for_family(key_id, PropertyFamily::Element, &value)?;
            Ok(BoundPredicate::Compare {
                key,
                key_id,
                op,
                value,
            })
        }
        LogicalPredicate::And(left, right) => Ok(BoundPredicate::And(
            Box::new(bind_predicate(*left, state)?),
            Box::new(bind_predicate(*right, state)?),
        )),
        LogicalPredicate::Or(left, right) => Ok(BoundPredicate::Or(
            Box::new(bind_predicate(*left, state)?),
            Box::new(bind_predicate(*right, state)?),
        )),
    }
}

/// Binds a logical operation against the catalog and lowers it to a plan.
///
/// This is the single seam where names resolve to catalog IDs and where
/// property literals are parsed and family/type-validated. Parsing above never
/// touches the catalog; execution below never resolves names.
fn bind_and_lower(op: LogicalOp, state: &impl StateView) -> Result<QueryPlan, DbError> {
    match op {
        LogicalOp::ElementScan => Ok(QueryPlan::ElementScan),
        LogicalOp::RelationScan => Ok(QueryPlan::RelationScan),
        LogicalOp::IncidenceScan => Ok(QueryPlan::IncidenceScan),
        LogicalOp::CatalogScan => Ok(QueryPlan::CatalogScan),
        LogicalOp::ElementLabelScan { label } => {
            let label_id = state
                .catalog()
                .label_id(&label)
                .ok_or_else(|| DbError::unsupported(format!("unknown label {label}")))?;
            Ok(QueryPlan::ElementLabelScan { label, label_id })
        }
        LogicalOp::RelationTypeScan { relation_type } => {
            let relation_type_id = state
                .catalog()
                .relation_type_id(&relation_type)
                .ok_or_else(|| {
                    DbError::unsupported(format!("unknown relation type {relation_type}"))
                })?;
            Ok(QueryPlan::RelationTypeScan {
                relation_type,
                relation_type_id,
            })
        }
        LogicalOp::ElementPropertyEqual { key, value } => {
            let key_id = state
                .catalog()
                .property_key_id(&key)
                .ok_or_else(|| DbError::unsupported(format!("unknown property key {key}")))?;
            let value = parse_value_token(&value).map_err(DbError::unsupported)?;
            state.validate_lookup_value_for_family(key_id, PropertyFamily::Element, &value)?;
            Ok(QueryPlan::ElementPropertyEqual { key, key_id, value })
        }
        LogicalOp::GraphWalk {
            projection,
            element,
            options,
        } => lower_graph_walk(&projection, &element, options, state),
        LogicalOp::ElementWhere { predicate } => Ok(QueryPlan::ElementFilter {
            predicate: bind_predicate(predicate, state)?,
        }),
    }
}

/// Resolves and validates an `OxQL` graph traversal into a physical walk.
fn lower_graph_walk(
    projection: &str,
    element: &str,
    options: Walk,
    state: &impl StateView,
) -> Result<QueryPlan, DbError> {
    let projection = state
        .catalog()
        .projection_id(projection)
        .ok_or_else(|| DbError::unsupported(format!("unknown projection {projection}")))?;
    let entry = state
        .catalog()
        .projection(projection)
        .ok_or(DbError::UnknownProjection { id: projection })?;
    if matches!(&entry.definition, ProjectionDefinition::Hypergraph(_)) {
        return Err(DbError::invalid_projection("projection is not a graph"));
    }
    let raw = element
        .parse::<u64>()
        .map_err(|_error| DbError::unsupported("element id must be an integer"))?;
    Ok(QueryPlan::GraphWalk {
        projection,
        element: ElementId::new(raw),
        options,
    })
}

/// Scans all elements.
fn scan_elements(state: &impl StateView) -> QueryResult {
    QueryResult::new(
        state
            .elements()
            .map(|record| QueryRow::single(QueryValue::Element(record.id)))
            .collect(),
    )
}

/// Scans all relations.
fn scan_relations(state: &impl StateView) -> QueryResult {
    QueryResult::new(
        state
            .relations()
            .map(|record| QueryRow::single(QueryValue::Relation(record.id)))
            .collect(),
    )
}

/// Scans all incidences.
fn scan_incidences(state: &impl StateView) -> QueryResult {
    QueryResult::new(
        state
            .incidences()
            .map(|record| QueryRow::single(QueryValue::Incidence(*record)))
            .collect(),
    )
}

/// Scans elements with one label.
fn scan_elements_with_label(state: &impl StateView, label_id: crate::LabelId) -> QueryResult {
    QueryResult::new(
        state
            .elements_with_label(label_id)
            .into_iter()
            .map(|id| QueryRow::single(QueryValue::Element(id)))
            .collect(),
    )
}

/// Scans elements with a property value.
fn scan_elements_with_property(
    state: &impl StateView,
    key: PropertyKeyId,
    value: &PropertyValue,
) -> Result<QueryResult, DbError> {
    Ok(QueryResult::new(
        state
            .typed_property_equal_for_family(key, PropertyFamily::Element, value)?
            .into_iter()
            .filter_map(|subject| match subject {
                PropertySubject::Element(id) => Some(QueryRow::single(QueryValue::Element(id))),
                PropertySubject::Relation(_) | PropertySubject::Incidence(_) => None,
            })
            .collect(),
    ))
}

/// Filters elements by a bound compound predicate.
///
/// # Performance
///
/// `O(elements × predicate nodes)`: one scan, evaluating the predicate per
/// element. A lone equality is lowered to the indexed property fast path
/// instead and never reaches this scan.
fn filter_elements(state: &impl StateView, predicate: &BoundPredicate) -> QueryResult {
    QueryResult::new(
        state
            .elements()
            .filter(|record| predicate.evaluate(state, record.id))
            .map(|record| QueryRow::single(QueryValue::Element(record.id)))
            .collect(),
    )
}

/// Scans relations with one relation type.
fn scan_relations_with_type(
    state: &impl StateView,
    relation_type: crate::RelationTypeId,
) -> QueryResult {
    QueryResult::new(
        state
            .relations_with_type(relation_type)
            .into_iter()
            .map(|id| QueryRow::single(QueryValue::Relation(id)))
            .collect(),
    )
}

/// Scans catalog entries.
fn scan_catalog(state: &impl StateView) -> QueryResult {
    let rows = state
        .catalog()
        .projections()
        .map(|entry| QueryRow {
            values: vec![
                QueryValue::Projection(entry.id),
                QueryValue::Text(entry.definition.name().to_owned()),
            ],
        })
        .collect();
    QueryResult::new(rows)
}

/// Executes a graph walk.
fn execute_graph_walk(
    state: &impl StateView,
    projection: ProjectionId,
    element: ElementId,
    options: Walk,
) -> Result<QueryResult, DbError> {
    let graph = graph_projection(state, projection)?;
    let subgraph = traversal::walk_graph_projection(&graph, &[element], options)?;
    let rows = subgraph
        .nodes
        .iter()
        .map(|node| QueryRow::single(QueryValue::Element(node.element)))
        .collect();
    Ok(QueryResult::new(rows))
}

/// Materializes a graph projection by ID.
fn graph_projection(
    state: &impl StateView,
    projection: ProjectionId,
) -> Result<GraphProjection, DbError> {
    let entry = state
        .catalog()
        .projection(projection)
        .ok_or(DbError::UnknownProjection { id: projection })?;
    match &entry.definition {
        ProjectionDefinition::Graph(definition) => {
            GraphProjection::from_state(state, definition.clone())
        }
        ProjectionDefinition::Hypergraph(_definition) => {
            Err(DbError::invalid_projection("projection is not a graph"))
        }
    }
}