Function convertPattern

packages/graph/src/astToSteps.ts:1377–1521

* Convert a pattern into a sequence of steps. * Handles node patterns, edge patterns, and variable-length paths. * * @param pattern The pattern to convert * @param whereClause Optional WHERE clause for filtering * @param anchoredToInput If true, the first node is a reference to an existing bind

(
  pattern: Pattern,
  whereClause?: WhereClause,
  anchoredToInput: boolean = false,
  preservePriorBindings: boolean = false,
)

 *                              to preserve variable bindings from a prior WITH clause.
 */
function convertPattern(
  pattern: Pattern,
  whereClause?: WhereClause,
  anchoredToInput: boolean = false,
  preservePriorBindings: boolean = false,
): Step<any>[] {
  const steps: Step<any>[] = [];
  const elements = pattern.elements;

  if (elements.length === 0) {
    throw new Error("Pattern must have at least one element");
  }

  // First element should be a NodePattern
  const firstElement = elements[0];
  if (firstElement?.type !== "NodePattern") {
    throw new Error("Pattern must start with a NodePattern");
  }

  const firstNode = firstElement as NodePattern;
  const stepLabels = firstNode.variable ? [firstNode.variable] : undefined;

  // Determine if this is an anchored pattern (variable reference to existing binding)
  // An anchored pattern has a variable but no labels and has more elements (edges)
  // In this case, we traverse from the input path rather than fetching all vertices.
  //
  // The heuristic only applies when anchoredToInput is explicitly true (passed from caller).
  // This handles cases like EXISTS { (n)-[:KNOWS]->(m) } where `n` is a bound variable.
  //
  // For fresh MATCH clauses, anchoredToInput is false, and we always create FetchVertices
  // even if the pattern looks like it could be anchored (e.g., MATCH (a)-[:knows]->(b)).
  // The label/property filter can come from the WHERE clause (e.g., WHERE a IS :Person).
  const isAnchorPattern =
    anchoredToInput &&
    firstNode.variable &&
    firstNode.labels.length === 0 &&
    !firstNode.labelExpression &&
    elements.length > 1 &&
    !firstNode.properties;

  if (!isAnchorPattern) {
    // Create initial fetch step for pattern match
    // Use CartesianFetchStep if we need to preserve bindings from a prior WITH clause
    if (preservePriorBindings) {
      steps.push(
        new CartesianFetchStep({
          vertexLabels: firstNode.labels.length > 0 ? firstNode.labels : undefined,
          stepLabels,
        }),
      );
    } else {
      steps.push(
        new FetchVerticesStep({
          vertexLabels: firstNode.labels.length > 0 ? firstNode.labels : undefined,
          stepLabels,
        }),
      );
    }