ClickHouse 26.3 Extends Vertical Merge to TTL Deletes, Cutting Memory Usage

ClickHouse 26.3 has expanded its Vertical Merge optimization to cover TTL DELETE merge operations, reducing memory consumption when expired rows are automatically removed. Background merges are a core part of ClickHouse's storage engine, continuously combining immutable data parts while applying maintenance tasks like TTL rules and recompression. Traditional horizontal merges process all columns simultaneously, causing peak memory usage to grow significantly for wide analytical tables with hundreds of columns. Vertical Merge addresses this by first resolving row ordering via primary keys and then processing each remaining column independently, keeping only a small subset of data active in memory at any time. This enhancement is particularly valuable for large production deployments storing complex, wide-schema datasets where background merges can become a major memory bottleneck.