ClickHouse's LowCardinality Column: One Dictionary, Millions of Rows
A LowCardinality column stores one byte per row instead of a whole string, because every block in a part points at the same dictionary — capped at 8192 keys and flushed when it …
Performance
ClickHouse's Query Condition Cache: Skip Granules You Don't Need to Read
ClickHouse caches the granules a predicate emptied, so a dashboard that runs the same query every 30 seconds stops paying for index analysis and skipped data.
ClickHouse's Query Result Cache: A Hash of the AST, Not the Text
ClickHouse caches query results by hashing the parsed query, not the text you typed, plus the settings and database that could change the answer.
ClickHouse's Variant Column Uses a Discriminator Byte and a Shared Fallback
A Variant column is a struct of arrays — one sub-column per alternative, one tag byte per row, one offset per row.
How ClickHouse Stores a Column: PODArray, the Typed Array, and the Padding Trick
Every ClickHouse column is one flat PODArray, padded 64 bytes on both sides.