SummingMergeTree and AggregatingMergeTree Collapse Groups in the Merge: No Hash Table, No Second Table
SummingMergeTree and AggregatingMergeTree combine rows inside the merge algorithm, group by group, as the sorted streams are walked.
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ClickHouse Incremental Backups Are Not Diffs: A Verified Byte Prefix and the Suffix After It
Incremental backups reuse the base backup’s bytes: only the suffix after a verified shared prefix is uploaded, and the restore concatenates the two halves.
ClickHouse Backs Up a Cluster with One Manifest: Sorting Gives Every File a Global Index
A cluster-wide backup is a ZooKeeper-coordinated election: one owner per data file, a stage barrier between phases, and a consensus loop over metadata.
ClickHouse Compiles a Date Format into a Program: Literal Text Becomes a memcpy Template
formatDateTime compiles its format string into an instruction vector and a byte template.
ClickHouse Runs Regular Expressions Four Ways: Literals Never Reach RE2
ClickHouse analyses a regexp before running it, and often never runs RE2 at all.