apache/druid · error · IllegalArgumentException

Cannot accept children

Error message

Cannot accept children

What it means

LeafDataSource is the base class for leaf datasources (table, lookup, query, inline) which by definition have no child data sources. Its final withChildren throws this IllegalArgumentException whenever a non-empty children list is supplied, enforcing the leaf invariant during datasource tree rewrites.

Solutions

1. Skip withChildren for leaf datasources (check DataSource.isLeafDataSource() or instanceof LeafDataSource) — returning the datasource itself is the correct behavior for empty children.
2. Fix the rewrite pass so it only calls withChildren on non-leaf (parent) datasources.
3. If your datasource truly has children, extend a non-leaf base instead of LeafDataSource.

Example fix

// before
for (DataSource ds : allDataSources) { ds = ds.withChildren(newChildren); }
// after
for (DataSource ds : allDataSources) {
  if (!ds.isLeafDataSource()) { ds = ds.withChildren(newChildren); }
}

if (!(ds instanceof LeafDataSource) && !children.isEmpty()) { ds = ds.withChildren(children); }

boolean isLeaf(DataSource ds) { return ds instanceof LeafDataSource || ds.isLeafDataSource(); }

try {
  ds = ds.withChildren(children);
} catch (IllegalArgumentException e) {
  if ("Cannot accept children".equals(e.getMessage())) { /* leave ds unchanged */ }
  else { throw e; }
}

When it happens

Trigger: Calling withChildren with any non-empty list on a table/lookup/inline/query datasource — typically a generic rewrite pass that calls withChildren uniformly on all datasources without checking isLeafDataSource().

Common situations: Custom datasource-rewriting/analysis code that pushes children into every node; new DataSource implementations accidentally extending LeafDataSource while carrying children; planner utilities that iterate all datasources calling withChildren.

Understand the failure class

Background: UnsupportedOperationException and "is not supported" errors: when a library deliberately refuses a call — this error's family across 30 libraries.

Related errors

• Cannot read directly out of dataSource
• Cannot retrieve map view from lookup
• Expected [1] child, got
• Expected [2] children, got
• Union datasource with non-leaf inputs is not supported

AI-assisted analysis of apache/druid@9b90983fd2 (2026-09-07). Data as JSON: /api/errors/f084ad58f8af25c7. Report an issue: GitHub.

Appendix: source

Thrown at processing/src/main/java/org/apache/druid/query/LeafDataSource.java:43

import java.util.Collections;
import java.util.List;

/**
 * Leaf {@link DataSource}-s have no inputs.
 */
public abstract class LeafDataSource implements DataSource
{
  @Override
  public final List<DataSource> getChildren()
  {
    return Collections.emptyList();
  }

  @Override
  public final DataSource withChildren(List<DataSource> children)
  {
    if (!children.isEmpty()) {
      throw new IAE("Cannot accept children");
    }

    return this;
  }

  @Override
  public SegmentMapFunction createSegmentMapFunction(Query query)
  {
    return SegmentMapFunction.IDENTITY;
  }
}

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