How ClickHouse Splits a Session Context from a Query Context
ClickHouse splits session state from per-query state, and gates every setting change through a constraint checker.
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How ClickHouse Turns Row-Level Security into a Filter Expression
ClickHouse mixes row policies into one boolean expression and injects it into the query plan as a filter on the table read.
How SummingMergeTree and AggregatingMergeTree Merge Rows
SummingMergeTree and AggregatingMergeTree combine rows inside the merge algorithm, group by group, as the sorted streams are walked.
How the ClickHouse Buffer Engine Holds Writes in RAM and Flushes Them in the Background
Buffer shards writes in RAM, flushes them on a min/max/flush threshold algebra, and restores the block if the destination write fails.
How the ClickHouse Kafka Engine Commits Offsets Only After the Block Is Written
Kafka offsets are committed only after the block has been written to the views, and a cancelled block is rewound to its start offset and replayed.