ClickHouse's LowCardinality Column Shares One Dictionary Across Millions of Rows
LowCardinality(T) stores a dictionary of distinct values once and keeps a 1–8 byte index per row.
Compression
How ClickHouse's Sparse Column Stores Only the Rows That Differ
Real columns are mostly default. A status column is 'active' for 98% of rows and '' for the rest. A retries column is 0 almost always. An amount column that is Nullable is NULL for …