databendlabs/databend · error

PartitionItem::BucketSpilled(_) => unreachable!()

Error message

PartitionItem::BucketSpilled(_) => unreachable!()

What it means

`AggregateMeta::serialize_mixed` panics when it encounters a `PartitionItem::BucketSpilled` entry. Mixed-mode serialization expects each partition to be either fully in-memory (Serialized) or an AggregatePayload; a bucket that was already spilled to disk must not be present in the data being serialized, so this indicates mixed-mode bookkeeping produced an invalid partition set.

Solutions

  1. Reduce memory pressure or disable/tune aggregation spilling settings so buckets do not spill unexpectedly in this code path
  2. Check whether the query uses features that mix spilled buckets with serialized payloads and avoid the affected combination
  3. Report a bug with the query and settings; the spill bookkeeping in aggregate_meta.rs needs to handle or reject BucketSpilled explicitly
  4. Upgrade Databend for spill-handling fixes in the aggregator

Example fix

// before
PartitionItem::BucketSpilled(_) => unreachable!(),
// after
PartitionItem::BucketSpilled(item) => return Err(ErrorCode::Internal(
    format!("BucketSpilled item {} not supported in serialize_mixed", item.bucket))),
Defensive patterns

Strategy: validation

Validate before calling

if data.iter().any(|i| matches!(i, PartitionItem::BucketSpilled(_))) { return Err(ErrorCode::Internal("BucketSpilled item in serialize_mixed input")); }

Type guard

fn is_serializable(i: &PartitionItem) -> bool { !matches!(i, PartitionItem::BucketSpilled(_)) }

Prevention

When it happens

Trigger: Serializing aggregate metadata that contains a `BucketSpilled` partition item — i.e. the aggregator spilled a bucket to disk but the spill/mixed-mode logic still handed the partition to `serialize_mixed`.

Common situations: Distributed aggregation with spilling enabled under memory pressure, where spill state and in-memory partition tracking get out of sync; bugs in spill-aware aggregation planning.

Understand the failure class

Background: "This is a bug, please report it": internal invariant violations, unreachable panics, and SNH errors explained — this error's family across 47 libraries.

Related errors

AI-assisted analysis of databendlabs/databend@288d84d76e (2026-09-11). Data as JSON: /api/errors/032d1fcc41decd39. Report an issue: GitHub.

Appendix: source

Thrown at src/query/service/src/pipelines/processors/transforms/aggregator/aggregate_meta.rs:227

                BinaryType::from_data(row_group_column),
            ]);

            return data_block.add_meta(Some(AggregateSerdeMeta::create_spilled(
                bucket_num as isize,
            )));
        }

        let mut buckets = Vec::with_capacity(data.len());
        let mut payload_row_counts = Vec::with_capacity(data.len());
        let mut payload_blocks = Vec::with_capacity(data.len());

        for item in data {
            let (bucket, block) = match item {
                PartitionItem::Serialized(payload) => (payload.bucket, payload.data_block),
                PartitionItem::AggregatePayload(payload) => {
                    (payload.bucket, payload.payload.aggregate_flush_all()?)
                }
                PartitionItem::BucketSpilled(_) => unreachable!(),
            };

            if block.num_rows() == 0 {
                continue;
            }
            buckets.push(bucket);
            payload_row_counts.push(block.num_rows());
            payload_blocks.push(block);
        }

        if payload_blocks.is_empty() {
            return Ok(DataBlock::empty());
        }

        let merged_block = DataBlock::concat(&payload_blocks)?;
        merged_block.add_meta(Some(AggregateSerdeMeta::create_partitioned_payload(
            buckets,
            payload_row_counts,

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