ClickHouse's Query Pipeline: Ports, Status Codes and the Executing Graph
ClickHouse doesn’t call operators in a fixed order. Every processor reports a status — ‘I need data’, ‘my output is full’, ‘call my work()’ — and the executor walks the…
Internals
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
Underneath every ClickHouse data type there is exactly one data structure: a flat array of trivially-copyable elements. UInt64 is an array of UInt64. String is a flat character …
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 …