Track · 2:30 · Liner note
A Projection of Calm
A ClickHouse projection is a precomputed, differently sorted or pre-aggregated copy of a table's data stored alongside it, which the query planner uses automatically when a query can be answered from it.
Some queries ask the same question all day: totals by day, counts by key. A projection answers it in advance. ClickHouse stores a pre-aggregated or differently sorted copy beside the table’s data, and the planner reaches for it when a query matches, with no change to the SQL. A Projection of Calm is the quiet that follows when the slow tile stops being slow.
Nothing is free. Every insert now maintains another copy, so storage and write cost rise. Add one for a query you have measured, not one you imagine, and confirm in the plan that it is actually used.