Fix #933 — bound compaction memory so wide-row tables don't OOM#942
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erikdarlingdata merged 1 commit intodevfrom May 7, 2026
Merged
Fix #933 — bound compaction memory so wide-row tables don't OOM#942erikdarlingdata merged 1 commit intodevfrom
erikdarlingdata merged 1 commit intodevfrom
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#935 added temp_directory so DuckDB could spill, but on wider workloads the working set still blew past the 4 GB cap before spill caught up (reporter saw OOM at 3.7 GiB compacting 15 query_snapshots files). Three knobs combined to feed that: - memory_limit = 4 GB was too high — DuckDB held off spilling until late - threads defaulted to N cores, multiplying per-thread row-group buffers - ROW_GROUP_SIZE 122880 buffered up to 122k wide-VARCHAR rows per group Drop memory_limit to 1 GB, cap threads to 2, and shrink ROW_GROUP_SIZE to 8192. On 1.7 M rows of real query_stats data this drops peak working set from 1236 MB → 166 MB (87% reduction) at a 31% wall-time cost. Memory now plateaus instead of growing with row count, which is the load-bearing change for issue #933. Adds tools/CompactionRepro — a standalone reproducer that splits a real monthly parquet file into N per-cycle-shaped chunks and runs the same pair-merge logic with the tuning knobs exposed on the command line. Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
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Summary
temp_directoryso DuckDB could spill during compaction, but on wider workloads the working set still blew past the 4 GB cap before spill caught up (reporter saw OOM at 3.7 GiB compacting 15query_snapshotsfiles).memory_limit = 4GBwas too high (DuckDB held off spilling),threadsdefaulted to N cores (per-thread row-group buffers multiplied), andROW_GROUP_SIZE 122880buffered up to 122k wide-VARCHAR rows per group.memory_limitto1GB, capthreads = 2, shrinkROW_GROUP_SIZEto8192. Memory now plateaus instead of growing with row count.Fixes #933
Repro tool
tools/CompactionRepro— standalone .NET console app that splits a real monthly parquet file into N per-cycle-shaped chunks and runs the same pair-merge logic with the tuning knobs exposed on the command line. Useful for validating future changes to compaction.Validation
On a real local archive (
202604_query_stats.parquet, 1.7M rows, ~70 MB):87% peak memory reduction. 31% slower wall time. Output 14% larger (smaller row groups → smaller compression dictionaries — acceptable trade for not crashing).
Test plan
🤖 Generated with Claude Code